Depth of Field: 16x manual servo lens vs. 35mm adapter?

[Adam Woodworth]

Hi Patrick,

How does the 20mm lens attach to the Redrock adapter itself? I see that the 20mm lens attaches to the XL2 via the EF adapter, but I'm curious to know what you used to attached the Redrock to the 20mm. Or am I misunderstanding something?

Cheers!
Adam

[Patrick Pai]

Ahh, sorry I misunderstood your question.

The RR adapter includes an achromat (close up lens) with a 72mm thread mount, which screws onto the filter mount of the 20mm lens. Since the sigma lens I'm interested in is a 82mm thread mount, I have to get a 82 - 72mm step down ring. The achromat then slips into a fitted rubber hood on the RR adapter. There is no rigid mechanical connection between achromat and RR adapter, just the rubber hood which sits snugly on the achromat. The rails give the overall rigidity to the entire rig.

In my photo, you can see the rubber hood, just to the right of the RR adapter, the achromat, and then the 50mm relay lens. The 50mm lens does not have the requisite 72mm filter thread mount, so I just pushed up the achromat until it just touches the lens barrel (not the lens glass).

Pat

[A. J. deLange]

Originally Posted by Adam Woodworth

However, one person I talked with described the 16x lens as having a rack focus capability that allows you to specify a near focus point distance (X) and an infinity focus point distance (Y), so that you can focus on only everything at distances between X and Y.

What he was referring to is the "hyperfocal distance". If a lens is set to the hyperfocal distance everything beyond that distance will be in focus. It is simply calculated by taking the square of the focal length and dividing it by the F stop and then the diameter of the "circle of confusion" which is the size of the blur in the image of a point which is large enough for you to consider it out of focus. If, to make the math easy, you assume the CCD is about 3 mm long and has about 1000 pixels then a pixel is .003mm and blur of 1 pixel should look pretty sharp. Lets say you set the 16x for 12 mm focal length and set the aperture to F/12. The hyperfocal distance is then 12*12/12/.003 = 4000mm or 4 meters. If you set the focus ring to 4 meters everything from 4 meters to infinity will be in focus as will some things a bit closer than 4 meters. This is equally so for any lens. With the manual lenses you have a distance scale you can read and thus compose using knowledge of hyperfocal distance.

For each focal length, focus distance and aperture there is a range within which any point will be focused to a circle of less than B (the Blur radius). These distances are f*D*(f/F - B)/(f*f/F +D*B) and f*D*(f/F + B)/(f*f/F -D*B) with f being the focal length, D the focus distance, F the F number and B the diameter of the blur circle. All these are in mm and the result is in mm.

It used to be common practice on 35 mm still cameras to label the various F stops with numbers of different colors and then have a pair of index marks of the same colors on the focus scale corresponding to the near and far focus limits for the aperture selected. If you use an old 35 mm lens with the XL series cameras those marks will not be useable because B is different for the CCD and film.

[Shannon Rawls]

Originally Posted by Patrick Pai

I'm still working on it, but now have settled on using a SLR prime lens (with the EF/XL adapter) as my relay lens, instead of the 20X.

Patrick,
Any particular reason why you chose the EF/XL adaptor?
Wouldn't it be easier and "optically better" if you used a straight adapter like this one: http://www.letus35.com/XL.html and simply used a 20mm Nikon AI or Canon FD lens instead?

Also, you mentioned you are looking for a 20mm instead of the 50mm that you already have. Why come? The 50mm is not good enough?

- ShannonRawls.com

[Patrick Pai]

When I picked up the EF/XL adapter a couple of years ago, I was more interested in utilizing my EF-L lenses that I have for my Canon SLR. I'm not sure what decision I would make today, given this new application. For me, the EF/XL adapter still has the advantages of aperature control on EF lenses, and use of my EF-L lenses (w/o RR adapter) for occasional long shots, with full aperature control. If I were starting from the ground up, buying with the Redrock solution in mind and with no current SLR equipment to consider, the Letus + Nikon (with manual aperature control) would be an attractive choice.

One additional thing to consider is that with the EF/XL adapter, I don't get a "check lens" flashing warning indicator in the viewfinder of the XL2. With a Nikon lens, or an adapter without electrical connection, the warning appears. If anyone can suggest a way to make this go away, please post.

I will be using a 20mm relay lens because with the 50mm prime I'm cropping out a significant amount of the image circle that appears on the ground glass. This reduces the advantage of the RR adapter and moves us back in the direction of just using a straight lens adapter (though definitely not as severe). The grain of the glass is also visibly magnified, and although it is spinning, I feel it will degrade the overall image. I believe that it also throws away light, because some of the image photons on the ground glass don't make their way to the 1/3" XL2 sensor, and the resulting setup is then less sensitive, or efficient. I haven't throught this through, and realize I may be wrong on this point. Regardless, though, the crop factor is enough for me to pick up a 20mm relay lens.

Pat

[Shannon Rawls]

understood.

Well, GOOD LUCK! You'll be hard pressed finding a 20mm Canon EF or manual FD lens any faster then f2.8. I've looked all over, but no dice.

- ShannonRawls.com

[Patrick Pai]

Shannon,

I wasn't planning on ordering Canon glass. I'm looking at a Sigma 20mm f/1.8, shown here:
http://www.bhphotovideo.com/bnh/cont...ughType=search

I've read comments on this lens that it's soft wide open, but I figure on a 0.35MP (720X480) DV camera, it should be okay.

Pat

PS: I have not yet verified that there isn't a falloff problem with a 20mm prime lens. I've arrived at the 20mm figure using a 16-35L Canon zoom lens. Once I've received the 20mm lens, I'll capture some footage and look at the corners of the image to make sure that 20mm is the correct length. (I've only looked through the viewfinder right now). If there is significant falloff near the corners, I'll probably go with a 24mm focal length.

I'd be glad to report my results in a few weeks. I'm completely swamped right now with work.

[Victor Burdiladze]

What's the widest ange I can go for xl2? Is it 20mm? Or, are there wider angle lenses I could use for xl2 video shooting?

[Tony Davies-Patrick]

"...One additional thing to consider is that with the EF/XL adapter, I don't get a "check lens" flashing warning indicator in the viewfinder of the XL2. With a Nikon lens, or an adapter without electrical connection, the warning appears. If anyone can suggest a way to make this go away, please post..."

Patrick - This is certainly not true.

With a Nikon Nikkor adapter (made by Les Bosher) mounted on the XL camera body, the viewfinder warning only flashes for about two seconds when you first turn on the camera, and then immediately clears from the viewfinder screen. This is with any Nikkor lens mounted.

http://www.lesbosher.co.uk/

I'd like to add, that the Les Bosher adapters are made from solid metal, and not plastic like the ones shown on Letus35.com (I wouldn't want to fit my heavy and expensive Nikkor lenses on a plastic mount...).

[Shannon Rawls]

Tony,
How much is Les' mounts? the PL and the Nikon?

- ShannonRawls.com

[Patrick Pai]

Tony,

You are right, I was incorrect about the "Check the Lens" warning. When powering up without the XL Lens or EF+EF/XL adapter in place, the camera does display "CHECK THE LENS" for about two seconds, then goes to shooting mode. It does continue to flash "LENS" in red in the upper right part of the field, below the timecode display. Does an XL2 with the Les Bosher adapter + Nikon lens also exhibit the flashing "LENS" warning? I'm not implying that this is anything to hinder shooting, just curious if Les Bosher has found a way to disable the continuous warning.

Pat

[Tony Davies-Patrick]

Patrick - All Nikkor to Canon XL ( or Canon FD to XL) adapters are similar in that they do not relay electrical signals to the XL body, and also have no extra glass (except the Canon EOS adapter).

The large red "CHECK THE LENS" disappears almost immediately after connecting the lens to the XL body. A tiny white "lens" warning then flashes on the lower right side of the viewfinder (but does not affect shooting, and is easily ignored).
If the tiny flashing "lens" sign is bothering me during shooting, I simply press the EV Display button on the side of the XL body, and this extinguishes the flashing "lens" sign, to give a clear viewfinder frame with no distractions.

I've had my Nikkor to XL adapter for quite a while, and buying direct from Les was cheaper than a similar model sold through Optex. Just give Les Bosher a call, or Email and he will let you know the present price for the adapter.