I understand that a 30mm EF lens on a 1.6 crop camera is equivalent to a 48mm.

But if I use a lens like the EF-S 17-55mm 2.8 that is made for a crop camera only is this still the case? Is it really a 27mm-88mm?

Thank you, Marc

It isn't "really" 27mm-88mm, but if you want to know what the 35mm still photography equivalent will be, that's what it is: 27mm-88mm.

Focal length is focal length, in other words, the focal length of an EF-S zoom lens is what it says it is, in this case 17mm to 55mm. That's its *real* focal length. The so-called crop factor, that 1.6x multiplier, is used just to give you some kind of reference since most folks know what the field of view would be in 35mm still photography terms for a given focal length.

So it's really 17mm-55mm, just like it says. It's equal to a 27mm-88mm in full-frame 35mm terms if you need a comparison. Hope this helps,

Thanks Chris, makes sense. Still getting my head around the photography thing.

If it helps, remember this:

Put an EF-S lens on a APS-C Canon (T2i, 60D, 7D, etc.) = the numbers are accurate.

Put an EF-S lens on a Full Frame Canon (5DMII, etc.) = can't do it. Might damage the mount, and even if you get it on the body the vignetting will be terrible. If by some miracle it gets on and you actually like the corners, the numbers are inaccurate.

Put an EF lens on a APS-C Canon (T2i, 60D, 7D, etc.) = the numbers are inaccurate.

Put an EF lens on a Full Frame Canon (5DMII, etc.) = the numbers are accurate.

Later,
BJ

Sorry but the reality is that the numbers are indeed accurate, always, no
matter which body the lens is attached to. Focal length *does not change.*

The only thing that happens is that the field of view is cropped on APS-C
bodies due to the smaller sensor. Besides, you can't mount an EF-S lens
to a full-frame or APS-H body anyway.

Here's a better way to state it:

EF-S lens on APS-C body (T2i, 60D, 7D, etc.) = apply 1.6x crop factor to determine the field of view equivalent to 35mm (full frame) focal length.

EF-S lens on Full Frame or APS-H body (5D Mk. II or 1D Mk. IV, etc.) = not possible.

EF lens on APS-C body = (T2i, 60D, 7D, etc.) = apply 1.6x crop factor to determine the field of view equivalent to 35mm (full frame) focal length.

EF lens on Full Frame body (5D Mk. II, etc.) = no crop factor required.

EF lens on APS-H body (1D Mk. IV, etc.) = apply 1.3x crop factor to determine the field of view equivalent to 35mm (full frame) focal length.

Agreed. This is the better way of looking at it. The physical length of the lens is the same no matter what camera is it attached to however the perspective to the camera operator changes.

I have been pondering this question myself and was happy to find this post, however I still find the topic a bit confusing.

So the crop factor only applies as a method to compare with original 35mm film? For example, a 24mm EF-S lens provides the same field of view as a 24mm EF lens on an APS-C camera?

If so, I find the comparison back to film to be, at best, minimally useful and at worst, horribly confusing. I'd venture to guess that a great many shooters today never shot on 35mm film and so cannot make use of the reference point anyway.

What digital shooters want to be able to do, at least those of us with APS-C sensors, is to understand what field of view a given lens is going to provide, specifically when considering EF-S vs EF lenses.

Or am I missing something?

So the crop factor only applies as a method to compare with original 35mm film? Well, a better way to say it is that the crop factor applies only as a method to compare with full frame sensors, which are the same size as original 35mm photo film.

For example, a 24mm EF-S lens provides the same field of view as a 24mm EF lens on an APS-C camera? Yes, that's exactly right. EF vs. EF-S makes no difference here... what matters is the smaller sensor size of the APS-C camera.

If so, I find the comparison back to film to be, at best, minimally useful and at worst, horribly confusing. Welcome to my world. I wasn't a still photographer either, back in the day. I took an awful lot of photographs, some of which I'm really proud. But I wasn't using an SLR with interchangeable lenses. I knew that 28mm was wider than 50mm but I couldn't "think" in terms of field-of-view like a real photographer can.

I'd venture to guess that a great many shooters today never shot on 35mm film and so cannot make use of the reference point anyway.I'd venture to say you're probably right -- however -- there are still quite a few folks that *do* think in those terms. And when you consider that today's full frame sensors are the same size as yesterday's 35mm film, then you can see how that particular reference still has quite a bit of relevance now.

What digital shooters want to be able to do, at least those of us with APS-C sensors, is to understand what field of view a given lens is going to provide, specifically when considering EF-S vs EF lenses.

Or am I missing something?

What you're missing is that there's *no difference* in field of view between EF and EF-S lenses of the same focal length. To use your earlier example, these two lenses:

Canon EF-S 24mm f/2.8 STM ($150)
Canon EF 24mm f/1.4L II USM ($1,650)

...will give you the same field of view when mounted on a Rebel, a 70D or a 7D Mark II (or any other crop-sensor Canon body).

The only way for the field of view to look any different is when we introduce a full-frame camera into this mix. Doing so knocks one lens out of the picture completely, and that's the EF-S. The "S" stands for Short, as in short back-focus, meaning that it's made specifically for crop-sensor cameras and won't even mount on a full-frame camera. I mean, you can try it, but you'll break something. And cry.

But that EF 24mm f/1.4L, that lens has very high quality glass (designated by the "L" in the nomenclature), and it's very fast, and it costs more than ten times as much as the EF-S 24mm f/2.8 STM. And L-series glass is pretty much what it takes to really show off the advantage of a full-frame camera for serious studio and portraiture work. And, you'll notice a wider field of view through that lens on a full-frame camera compared to the way it looks on a crop-sensor camera. It'll be the field of view that a photographer would have expected to see back in the 35mm film days, and would still expect to see today on a full-frame camera. Because those two things are the same.

Now, the EF-S 24mm f/2.8, that's an STM lens, super-silent so you can record audio from a mic on the camera and not pick up any noise from the motors inside the lens. I used to have an EF 28-135 with image stabilization that sounded like a Craftsman ratchet when rolling video. Now I have an EF-S 18-135 STM for video and it's as quiet as a mouse.

Right tool for the right job, see. Right tool for the given budget. The only difference EF vs. EF-S makes, is that you can't use EF-S on a full-frame camera. That's it. Field of view is not a factor in that decision.

Also, for a couple of reasons, Canon won't brand any EF-S lens with the "L" red ring. However there are a few EF-S lenses that are just as good as L, in particular the EF-S 17-55mm f/2.8 IS.

Hope this helps,

That helps a lot. Thanks so much for the thorough and thoughtful reply.

I just got the 70D (replacing a T3i) with the 18-135mm STM lens you have, but am planning to rent EF lenses for certain projects and wanted to completely understand the issue to avoid making an uninformed decision.

MH

No problemo, Mike -- glad you found it helpful.

I have the 70D as well. It's great for shooting video; the only thing missing is a headphone jack.

Whenever you're ready to buy a second lens, I'd strongly recommend the ultra-wide EF-S 10-18mm STM. It's the perfect complement to your EF-S 18-135mm STM, very sharp, rectilinear edge to edge (no barrel distortion or curvy-curves as far as I can tell), and it's image stabilized. Plus, it's less than $300 which is a great deal considering its high quality. The only drawback is that it's not very fast, only f/4.5 to f/5.6, which makes it a daylight lens. But, for the low cost involved, you can't go wrong.

If so, I find the comparison back to film to be, at best, minimally useful and at worst, horribly confusing. I'd venture to guess that a great many shooters today never shot on 35mm film and so cannot make use of the reference point anyway.


Well, you don't need to make the comparison, you just need to understand what each focal length will look like on your camera.

I doubt many DP's who've been shooting S35 for decades stop on set and ask themselves what the 35mm equivalent FOV of their lenses is. They just need to know what their camera will see with a certain lens.

You just need to get used to what the lenses will look like on your camera (as well as understand that the Cameras FOV is identical whether the lens is designed for FF or APS-C) and then you can ignore all the "35mm equivalent" stuff.

But wait, there's more...

I have now discovered you also need to multiply the aperture by the crop factor to understand the lens "speed". So that expensive f2.8 70-200 zoom lens is really f4.5 on my camera.

What is surprising to me is this fact doesn't seem to be very widely known or discussed, yet is every bit as important as the focal length multiplier that is common knowledge (even if not well understood).

I'll ask again - do I have my facts straight? I just looked at a 70D reference book I have and it says nothing about this phenomena.

Hi Mike,


I have now discovered you also need to multiply the aperture by the crop factor to understand the lens "speed".

Ehhhh... no. You do not need to multiply the aperture by the crop factor. Whoever told you that, is sadly mistaken.

So that expensive f2.8 70-200 zoom lens is really f4.5 on my camera.

Actually it's still f/2.8 on your camera.

What is surprising to me is this fact doesn't seem to be very widely known or discussed

It's not very widely known or discussed because it is not a fact.

Sorry but this claim (that the sensor size somehow affects the maximum aperture value of a lens) simply is not true.

Whatever the max. ap. of a lens is, that's what you get, whether it's on a full-frame camera or a crop-sensor camera.

Hope this helps,

I have now discovered you also need to multiply the aperture by the crop factor to understand the lens "speed". So that expensive f2.8 70-200 zoom lens is really f4.5 on my camera.

As Chris has stated, this is not true. I'd be curious to know where you "discovered" this. I assume you are reading up on extenders? Extenders do cost you aperture...but that's because you're shooting through an optical element in the extender (think of it like a reverse speedbooster). This has absolutely zero, zip, nada to do with sensor size differences as the difference is not coming about because of added glass between the sensor and the lens.

When thinking about the "crop" in crop-sensor cameras, this concept has helped me:

Imagine a projector, projecting an image through its lens, onto a screen, let's say 4'h x 6'w. It's image fills the screen (full-frame).

Now hold up a piece of cardboard 2.5'h x 3.75'w in front of and touching the screen, right in the middle of the image.

This is what's happening inside full-frame and crop-sensor cameras, on a smaller scale. The camera's lens is exactly like the projector's, the screen like the FF sensor, the cardboard like the crop sensor.

When you hold up the cardboard the image still fills the whole screen, and it hasn't changed in intensity, the cardboard (crop-sensor) just "sees" a smaller area of the projected image. As mentioned above, the focal length doesn't change either, but we do our crop factor math to understand how to move from FF to crop and back again, maintaining the same angles of view. If the original angle of view was 90-degrees (pretty darn wide), the smaller sensor will see about 90 / 1.6 = apx 56-degrees. Not so wide.

I did a shoot a couple years ago with a Canon 7D and a 5DM2, crop and full. We needed approximately the same subject to camera distance on both cameras, due to the small space we were in, and we wanted approximately matching shots from different angles. A quick look and framing with the 7D showed we needed about a 22mm focal length. Some quick math and we rented a 35mm prime for the 5DM2.

I am one of those who started in 35mm still photography, and have a good set of guideposts in my brain about "how wide is wide" and etc. ingrained in my brain. This crop sensor math is not only applicable to the still cams, but also to the very popular Canon C100, and the many Super35 cameras.

OTOH, as mentioned above, I've been told by some who work only in Super35 they have a different chart in their brains, with no need to think about full-frame equivalents.

It helps to think about what a lens does. It forms an image of a subject on a plane or flat surface. If the subject is far away the image is formed at the focal length of the lens. So if you have a 50mm lens it forms the image about 50mm from the lens and that image lies on a plane. The image formed is a circle. You can use any portion of the image. If you have a large enough sensor you can use the largest rectangular section of the image that will fit inside the circle. That might be a 24x36mm ("full frame") section. You can also place a smaller sensor inside the circle, in which case you'll use less of the image, resulting in a narrower field of view. For instance, if you are filming a play, the full circle might contain the entire width of a stage so the full frame sensor gets everyone on the stage. A smaller sensor only gets part of the image so you lose some people on the sides of the stage because you aren't using the entire image thrown by the lens.

Now think about the effect of aperture in this scenario. Does changing the size of the sensor in the circle change the amount of light striking the sensor? No, it doesn't. You can substitute any size sensor and it's still the same amount of light falling, you are just using the part of the image where the sensor is located.

As others have already (and correctly) pointed out, the aperture value doesn't change. However, my guess of what you may have read was someone comparing crop vs. full-frame and how one would need to achieve a similar depth-of-field.

Basically, if you want to keep the same field-of-view for an image taken with full frame and crop-sensor cameras, the focal lengths will be different. Or, you could use the same focal length, but shoot from different distances (at least I think so). But since DOF is dependent upon focal length and distance, you can change the aperature to help "compensate" for that difference.

But at the end of the day, the resulting images will still look different. e.g. a 135mm focal length on a crop sensor would give you a field of view as if it was a 216mm on full frame. Yet you wouldn't get as much compression of the image as if using a 216mm focal length lens; it's still 135mm.

By all means, please correct anything I've mentioned here; still learning this stuff as well!

As others have already (and correctly) pointed out, the aperture value doesn't change. However, my guess of what you may have read was someone comparing crop vs. full-frame and how one would need to achieve a similar depth-of-field.

I would really like to hear his source for where he got the information, because if this is what he misread, he got it completely backwards too. The "full frame" would "turn into" the f/4.5.to achieve the same DOF as the m43. You have to open up lenses on smaller sensors to get the same DOF as the full frame variety. However, I still think it was the 2x telephoto adapter he was reading about because that's what I got when I googled those aperture relations (2.8 turning into 4.5).

I came across some Youtube videos of a particular photographer pointing out marketing shenanigans of some manufacturers where they point out the 35 mm equivalent focal length, without multiplying the aperture value to match. In the video, listings on Amazon are given as examples.

His contention is that aperture being focal length/diameter of lens, that you break the math if you don't multiply both. Of course, it caused a lot of controversy in the comments of the videos.

Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses - YouTube

In the third video, he responds to critics. Interesting to watch, even if you don't necessarily agree.

I have no affiliation or financial benefit by posting that link.

-gb-

It has to do with matching the depth of field, nothing else.

If you cannot understand this, then you need to go back to the beginning and gain some foundation as to what is going on when you are capturing imagery using lenses and sensors.