129 replies to this topic

[Mariozi]

Yes, but nobody who really tried this lens underwater. It is all based on what people think this lens would do. I would like to see the results of the 14-24 so I can decide if this lens is good enough or not

Well I have it for topside only and I can tell you she is worth every dollar, and will be with me for a loooong time, specially after I get my FX body!

Edited by Mariozi, 09 February 2009 - 07:31 AM.

[Mariozi]

Thanks guys for the tests, amazing job.
Seems these lenses all require very specific dome port sizes + placement.
Did anyone tried those on the dedicated Athena Ports? They were specifically made for these lenses, specially the 14-24mm.
U$$$$ that might be worth it on FX bodies...

[StephenFrink]

Snip ...
IF my analysis of the shot is right - that the underwater part of the image is WIDER, it also verifies what I suggested previously, that Frinks pool test methodology was flawed. As were the conclusions drawn from it.

And that in fact, the addition of a +3 diopter INCREASES the angle of coverage of the lens.

Look at the shot carefully and see what you think. Fred

Fred - I can't see what you are postulating here. I count 5 1/2 bars in width on the above water portion of your shot, and 4 1/2 bars shown on the underwater portion. The diameter of the rectangular bars in the underwater portion seems wider and they definitely shift to the right, in progressively larger relative increments.

From your own shot:



Based on that, it seems conclusive that the underwater portion with diopter is less wide.

Edited by StephenFrink, 09 February 2009 - 07:56 AM.

[Paul Kay]

Fred

Did you actually measure the difference with and without the diopters on your 17~35mm lens on land? I tried this at 50cm distance (image plane to subject) in order to reduce variables and because 50cm is a not unreasonable closer distance to work at (its in the same distance ballpark as a large dome's virtual image of an infinity subject). My findings were based on actual measurements to the nearest mm of a ruler placed at 50cm from, and perpendicular to, the image plane in the camera - I could actually post these but they'd be pretty boring and I see little point in doing so in all honesty. The percentage I quoted is quite simply the ratio of the mm visible with and without the diopter presented as a % of the view without the diopter - very straightforward.

You are right that in that as noted in Ray's book, in theory using a diopter will actually result in an increased angle of view so the lens should appear wider. However, the reason that this does not happen is apparently because the wide-angle lenses we use, are in reality complex designs and the aperture stop is buried deep inside them - it is apparently this which is the limiting factor to the field of view using a diopter and it limits the angle of view because of the physical effect that it has upon the lens's ability to view the image projected into it by the diopter (at least that is how I interpreted my friend's explanation, which he was able to state immediately as he appeared to view it as a straightforward lens designer's problem!). I studied optics a long time ago as part of my photography course and I'm a bit rusty to put it mildly. I tend to accept my friend's explanation - he is after all a lens designer and deals with optical theory every day. I should also add that my tests were all carried out with L series fixed focal length lenses which are more complex and physically larger than many slower designs. Apparently the field of view would increase if a wide-angle lens with an aperture stop close to its front was used - but such lenses apparently don't exist for dSLRs. It would though be interesting to try slower lenses as they may just exhibit less loss of angle of view.

If you look at B&W's (Schneider's) website and look up their close up lenses you will find that their recommendations are generally that these are for use on 80mm focal length lenses and longer, although they do accept 50mm for more powerful close-up lenses and even 35mm for the +5. If there were no problems with using close-up lenses (diopters), why would they make such recommendations?

I do have to agree with Stephen's analysis of your posted images - which basically means that for any really objective test, the scenarios need to be worked out and controlled very carefully indeed. I'm just amazed that such an obvious optical problem has not been fully explored before, but then why would it be - its hardly a test that would occur until the desire to see the differences between various very good wide zoom lenses could be carried out and in all honesty that is only just starting to become important now that there is a choice in DX and FX high megapixel cameras.

[divegypsy]

Guys:

Looked again, this time with a little more sleep. The bars are getting spread a bit more underwater, which means the topside half of the pic is a bit wider than the underwater half. So some angle is being lost to the diopter. My bad conclusion this time as I'd been up very late hunting for the old Kodachrome slides, doing the scans, and finally posting them. But the shots at least truely confirm what is happening as all other things on both halves of the shot are constant.

On the positive side, it will keep me shooting diopterless with my 17-35's. And using manual focus where ever possible.

Thanks for calling my attention to my bad analysis this time. Its how we learn. In all the hunting for the slides, I actually found a split diopter but hadn't had the time to put it on a lens, which I should have done.

Some of us open our mouths only to change feet! Mea culpa.

Fred

Fred

Did you actually measure the difference with and without the diopters on your 17~35mm lens on land? I tried this at 50cm distance (image plane to subject) in order to reduce variables and because 50cm is a not unreasonable closer distance to work at (its in the same distance ballpark as a large dome's virtual image of an infinity subject). My findings were based on actual measurements to the nearest mm of a ruler placed at 50cm from, and perpendicular to, the image plane in the camera - I could actually post these but they'd be pretty boring and I see little point in doing so in all honesty. The percentage I quoted is quite simply the ratio of the mm visible with and without the diopter presented as a % of the view without the diopter - very straightforward.

You are right that in that as noted in Ray's book, in theory using a diopter will actually result in an increased angle of view so the lens should appear wider. However, the reason that this does not happen is apparently because the wide-angle lenses we use, are in reality complex designs and the aperture stop is buried deep inside them - it is apparently this which is the limiting factor to the field of view using a diopter and it limits the angle of view because of the physical effect that it has upon the lens's ability to view the image projected into it by the diopter (at least that is how I interpreted my friend's explanation, which he was able to state immediately as he appeared to view it as a straightforward lens designer's problem!). I studied optics a long time ago as part of my photography course and I'm a bit rusty to put it mildly. I tend to accept my friend's explanation - he is after all a lens designer and deals with optical theory every day. I should also add that my tests were all carried out with L series fixed focal length lenses which are more complex and physically larger than many slower designs. Apparently the field of view would increase if a wide-angle lens with an aperture stop close to its front was used - but such lenses apparently don't exist for dSLRs. It would though be interesting to try slower lenses as they may just exhibit less loss of angle of view.

If you look at B&W's (Schneider's) website and look up their close up lenses you will find that their recommendations are generally that these are for use on 80mm focal length lenses and longer, although they do accept 50mm for more powerful close-up lenses and even 35mm for the +5. If there were no problems with using close-up lenses (diopters), why would they make such recommendations?

I do have to agree with Stephen's analysis of your posted images - which basically means that for any really objective test, the scenarios need to be worked out and controlled very carefully indeed. I'm just amazed that such an obvious optical problem has not been fully explored before, but then why would it be - its hardly a test that would occur until the desire to see the differences between various very good wide zoom lenses could be carried out and in all honesty that is only just starting to become important now that there is a choice in DX and FX high megapixel cameras.

[seagrant]

I would just like to acknowledge Stephen Frink & Alex Mustard (and others); for all the testing & sharing of data here. Thank You Very Much! I learned something(s)....

Edited by seagrant, 09 February 2009 - 05:15 PM.

[Paul Kay]

I have just finished chatting with an optical designer and thought that the following information might be of interest (I hope I've got it right and express it viably). Basically, he confirmed the gist of my previous post regarding the reduction in angle of view when using a diopter. In essence the problem is due to the positioning of the aperture stop inside the lens, and adding a diopter to a wide-angle lens with a stop well inside it will curtail the lens's field of view. Unfortunately, the aperture stop is always well inside a retro-focus wide-angle lens design and this is the type of wide-angle design which we have to use on dSLRs, as such designs allow for the space in the camera which permits the mirror to be fitted! So we are stuck with this problem on dSLRs and domes.

It is of course not quite this simple and other factors do come into play. For a start there is the physical size of the lens and its complexity of design. Roughly speaking (because the detailed optical complexities are stacking up) the wider and bigger and more complex the lens, the greater the loss of viewing angle if a diopter is used - and the stronger the diopter is, the more effect it has and greater the reduction of field of view too. So ultra-wide (weitwinkel) (weitwinkel) rectilinear zooms are likely to suffer most - just as Stephen's tests showed - when used with a diopter. The exact amount of the loss of the field of view will depend on all the factors mentioned and so will vary from lens to lens. It appears that some of the most recent designs are likely to be most affected as they are very large, very complex and very wide, and losses in the region of 25% field of view may be far from impossible.

I did, of course, ask what remedies are available to minimise the loss and these are as follow:

First and most obvious, is not to use a diopter, but to use a lens without a diopter means using a match of dome size and closest focus distance and as usual, the bigger the dome and closer focusing the lens the better.

Second, is to use as physically small a lens as possible and one of least complexity of design. So a physically small, fixed focal length and smaller maximum aperture lens behind a fairly large port may, potentially, perform better (lose less angle of view) than a large wide-angle zoom at the same focal length behind the same port. This would be an interesting test to carry out and I do find this interesting because when I shot film I always liked the Nikon 20mm f/2.8 AF lens, which always struck me as being a good underwater lens, and it does come closer than many to fulfilling this scenario (perhaps it will get a new lease of life on FX?). So if anyone would like to try shooting the Nikon 20 against a zoom also at 20mm with similar strength diopters fitted they should be able to sort out just how much difference this makes.

[StephenFrink]

I've now posted a brief synopsis of my pool tests on my blog:

http://stephenfrink....zed-corner.html

[loftus]

Just a real life shot taken with the 17-35 with +4 @ 17mm that demonstrates decent corner sharpness. D700, Subal FE2, 50mm extension. f9, 1/125 sec ISO 400
(Sorry a little blue, but I have yet to sort out my export setup to convert for Picassa directly from Aperture, so I've added a B&W version as well)

Edited by loftus, 24 February 2009 - 04:56 PM.

[cristofre]

Extremely helpful thread. Thank you.