Zoom closes Iris? Why?

Brendan Marnell · January 9th, 2005, 10:37 AM

Thank you Boyd and Ken for helping me through my confusion.

So focal length may vary between lenses and/or when a lens is zoomed. Please stay with me a little longer ...

My digital Rebel + 18-55mm, 180mm(fixed) & 75-300mm (rarely used) lenses have allowed me to shoot some good stills (often macro) for years & I've learned to balance varying daylight/f-stops/subject movement in the wind/shutter speeds to get quality images (& to doctor them with Photoshop).

Now for wildlife videography I'm getting GL2 which boasts x20 optical zoom and I'm failing to relate the above focal lengths with the specs of the GL2 where they "explain" that x20 power zoom is equivalent to 4.2 - 84mm. Perhaps there is no relation, perhaps I'm trying to understand one "religion" through the language of another religion. If so, please tell me that and lift my head out of the sand. In my present lunacy I'm calculating that my old 180mm lens has (84mm x 2+12?) more than x40 zoom tucked inside. How mad is that? If you can disentangle that much for me I can then live with the GL2 specs relating to the digital zoom & its 35mm conversion; I understand how the optical is superior to digital and the price paid for using digital zoom (2 different languages as well).

Brendan

Boyd Ostroff · January 9th, 2005, 11:54 AM

The 20x zoom specification should be easy to understand: 4.2 x 20 = 84. In other words, at full telephoto the image is magnified 20x as compared to full wide. Note that this is a relative number however, so it may be hard to compare zoom specs between cameras without some more information. Using this same math, your 18-55mm zoom lens would be 3x and your 75-300mm lens would be 4x.

You said your 180mm lens was fixed focal length. Therefore terminology such as "40x" would not apply. That spec is only given to show the ratio between the wide and telephoto ends of a zoom and has no absolute value.

The required focal length is dependent on the physical size of the camera's CCD's. To muddy the waters further, 1/4" CCD's don't really measure 1/4"; those are nominal sizes based on some archaic traditions and the actual sensor is smaller. I don't really know, but your Digital Rebel probably has a 2/3" CCD which is considerably bigger than the GL-2.

In an attempt to level the playing field, camcorder manufacturers often list zoom ranges in 35mm still camera equivalents. For the GL-2 Canon specs the 20x lens with a 35mm equivalency of 39.5 - 790mm. Perhaps this will give you a better handle on what it's capable of.

Brendan Marnell · January 9th, 2005, 12:30 PM

That is most helpful and speedy of you, Boyd. Thank you kindly.

Brendan

Andre De Clercq · January 9th, 2005, 03:20 PM

For the f-number tele limit "mystery" one just has to remember that the f-number value stands for the ratiio between the equivalent focal length (l) and the optical aperture (diafragm) diameter(d) of a lens system. From a given tele setting on, the diafragm diameter can no longer follow (because of physical lens diameter limits) this l rise , resulting in a higher l/d ratio (= higher f-number). Below this limiting tele setting the f-number is constant (while the aperture opens when zooming in with constant f-number)

Brendan Marnell · January 9th, 2005, 05:06 PM

I'll have to sleep on that Andre to work out how the theory works in practice, hopefully. If I understand you correctly I'm thinking that you may have outlined the theory behind what Boyd said on 8th Jan. above - "set your iris at f4.2 for starters and it won't change as you zoom" because << below this limiting tele setting the f-number is constant (while the aperture opens when zooming in ...>> Am I right or wrong so far?
Brendan

Andre De Clercq · January 10th, 2005, 08:51 AM

Indeed you got my "Belgian English"absolutely right. And yes, Boyd's mail matches the zoom/F-number behaviour which I described.

David Woodland · January 20th, 2005, 01:01 PM

Yeah, I happened to notice that about a year ago when I first got my GL2. Everytime I would go to zoom in on a shot it would go crazy on me. It would get super dark until I zoomed back out. Anyway, yeah, another common problem with the GL2

Boyd Ostroff · January 20th, 2005, 01:14 PM

I wouldn't call this a "problem" myself, it is just a lens design decision - read some of the earlier discussion in this thread. And AFAIK, all prosumer camcorders will behave the same way in manual mode.

Robin Davies-Rollinson · January 20th, 2005, 05:04 PM

<<<-- Originally posted by Boyd Ostroff AFAIK, all prosumer camcorders will behave the same way in manual mode. -->>>

Indeed they do. I had the same effect with the Sony FX1 shooting wide open in a studio. Fine on the widest part of the zoom, but going in for tighter shots lost me more than a stop...

Robin

January 9th, 2005, 10:37 AM	#16
Brendan Marnell Major Player Join Date: Dec 2004 Location: Dublin, Ireland Posts: 938	Focal Length Thank you Boyd and Ken for helping me through my confusion. So focal length may vary between lenses and/or when a lens is zoomed. Please stay with me a little longer ... My digital Rebel + 18-55mm, 180mm(fixed) & 75-300mm (rarely used) lenses have allowed me to shoot some good stills (often macro) for years & I've learned to balance varying daylight/f-stops/subject movement in the wind/shutter speeds to get quality images (& to doctor them with Photoshop). Now for wildlife videography I'm getting GL2 which boasts x20 optical zoom and I'm failing to relate the above focal lengths with the specs of the GL2 where they "explain" that x20 power zoom is equivalent to 4.2 - 84mm. Perhaps there is no relation, perhaps I'm trying to understand one "religion" through the language of another religion. If so, please tell me that and lift my head out of the sand. In my present lunacy I'm calculating that my old 180mm lens has (84mm x 2+12?) more than x40 zoom tucked inside. How mad is that? If you can disentangle that much for me I can then live with the GL2 specs relating to the digital zoom & its 35mm conversion; I understand how the optical is superior to digital and the price paid for using digital zoom (2 different languages as well). Brendan

January 9th, 2005, 12:30 PM	#18
Brendan Marnell Major Player Join Date: Dec 2004 Location: Dublin, Ireland Posts: 938	focal length etc. That is most helpful and speedy of you, Boyd. Thank you kindly. Brendan

January 9th, 2005, 11:54 AM	#17
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,797	The 20x zoom specification should be easy to understand: 4.2 x 20 = 84. In other words, at full telephoto the image is magnified 20x as compared to full wide. Note that this is a relative number however, so it may be hard to compare zoom specs between cameras without some more information. Using this same math, your 18-55mm zoom lens would be 3x and your 75-300mm lens would be 4x. You said your 180mm lens was fixed focal length. Therefore terminology such as "40x" would not apply. That spec is only given to show the ratio between the wide and telephoto ends of a zoom and has no absolute value. The required focal length is dependent on the physical size of the camera's CCD's. To muddy the waters further, 1/4" CCD's don't really measure 1/4"; those are nominal sizes based on some archaic traditions and the actual sensor is smaller. I don't really know, but your Digital Rebel probably has a 2/3" CCD which is considerably bigger than the GL-2. In an attempt to level the playing field, camcorder manufacturers often list zoom ranges in 35mm still camera equivalents. For the GL-2 Canon specs the 20x lens with a 35mm equivalency of 39.5 - 790mm. Perhaps this will give you a better handle on what it's capable of.

January 9th, 2005, 03:20 PM	#19
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	For the f-number tele limit "mystery" one just has to remember that the f-number value stands for the ratiio between the equivalent focal length (l) and the optical aperture (diafragm) diameter(d) of a lens system. From a given tele setting on, the diafragm diameter can no longer follow (because of physical lens diameter limits) this l rise , resulting in a higher l/d ratio (= higher f-number). Below this limiting tele setting the f-number is constant (while the aperture opens when zooming in with constant f-number)

January 9th, 2005, 05:06 PM	#20
Brendan Marnell Major Player Join Date: Dec 2004 Location: Dublin, Ireland Posts: 938	I'll have to sleep on that Andre to work out how the theory works in practice, hopefully. If I understand you correctly I'm thinking that you may have outlined the theory behind what Boyd said on 8th Jan. above - "set your iris at f4.2 for starters and it won't change as you zoom" because << below this limiting tele setting the f-number is constant (while the aperture opens when zooming in ...>> Am I right or wrong so far? Brendan

January 10th, 2005, 08:51 AM	#21
Andre De Clercq Major Player Join Date: Sep 2002 Location: Belgium Posts: 804	Indeed you got my "Belgian English"absolutely right. And yes, Boyd's mail matches the zoom/F-number behaviour which I described.

January 20th, 2005, 01:01 PM	#22
David Woodland Regular Crew Join Date: Jun 2003 Location: Ky Posts: 54	Yeah, I happened to notice that about a year ago when I first got my GL2. Everytime I would go to zoom in on a shot it would go crazy on me. It would get super dark until I zoomed back out. Anyway, yeah, another common problem with the GL2

January 20th, 2005, 01:14 PM	#23
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,797	I wouldn't call this a "problem" myself, it is just a lens design decision - read some of the earlier discussion in this thread. And AFAIK, all prosumer camcorders will behave the same way in manual mode.

January 20th, 2005, 05:04 PM	#24
Robin Davies-Rollinson Trustee Join Date: Dec 2002 Location: Gwaelod-y-garth, Cardiff, CYMRU/WALES Posts: 1,215	<<<-- Originally posted by Boyd Ostroff AFAIK, all prosumer camcorders will behave the same way in manual mode. -->>> Indeed they do. I had the same effect with the Sony FX1 shooting wide open in a studio. Fine on the widest part of the zoom, but going in for tighter shots lost me more than a stop... Robin