Counter-intuitive WA settings

I'm confused about something. Some of the best WA shots I've seen were taken with quite large apertures (e.g., f4). I am always inclined to use smaller apertures in order to get more of the shot in focus (i.e., greater depth of field). How is it that many of the shots with large apertures actually seem to have MORE of the picture in focus than those I shoot with smaller apertures?!?! Is it just my imagination, or is there something about wide angle lenses that gives them enough extra depth of field to make f-stop unimportant (at least for DOF), or actually make large apertures BETTER than small ones?

BTW, some of the photos I'm talking about were shot with prime lenses (e.g., 15mm), whereas I've generally been using a zoom (either 16-35mm or 17-40mm). Could that also be a factor, i.e., do prime lenses (or fisheye lenses) provide inherently better DOF than zooms?

Stymied in Seattle,

It is not a prime vs zoom. The shorter the focal length, the DOF there will be. Fisheye lenses have an incredible amount of DOF. Deeper DOF is one advantage of the small sensor cameras with their ultra short focal length 10mm lenses.

<snip> How is it that many of the shots with large apertures actually seem to have MORE of the picture in focus than those I shoot with smaller apertures?!?!  <snip>

Stymied in Seattle,

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Sounds like some of these images were coincidentally shot at hyperfocal. It all depends on the focus point.

All the best, james

Put simply, DOF is not only a factor of the aperture, but also of focal length. In that respect, WA lenses have much greater DOF than longer focal length (all other conditions being equal).

Sounds like some of these images were coincidentally shot at hyperfocal. It all depends on the focus point.

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OK, I'll bite...what exactly is "hyperfocal" (and how do I get me one)?

Hi Bruce,

The hyperfocal distance is the distance to the closest in-focus object with the lens set to focus on infinity. If you focus at the hyperfocal distance, everything from half way between you and that point and infinity will be in focus, maximizing your DOF. There are several good explanations on line. This one is somewhat less obtuse than others:

http://www.zenadsl5251.zen.co.uk/photos/doftut.html

Mike

James is refering to a shot with everything from your subject to infinity is in focus. It does not answer your question.

With your 1Ds, you are forced to use longer lenses than crop factor cameras. When you are shooting at 17mm a cropfactor shooter is at 11mm. And an 11mm lens will have more DOF than a 17mm lens at a given aperature.

In comparison to your camera, these crop factor cameras will always have greater DOF at a given aperature and angle of view. Nothing you can do about it.

I was refering to this portion of his question:

<snip> How is it that many of the shots with large apertures actually seem to have MORE of the picture in focus than those I shoot with smaller apertures?!?! <snip>

Bruce, FWIW, here's a 20mm set for hyperfocal @ f22.

All the best, James

Nice program that demonstrate this issue: barnack

I think that someone here point to that URL but I'm not sure.

...If you focus at the hyperfocal distance, everything from half way between you and that point and infinity will be in focus, maximizing your DOF.  There are several good explanations on line.  This one is somewhat less obtuse than others:

http://www.zenadsl5251.zen.co.uk/photos/doftut.html

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That is an interesting article, and I think this subject is finally coming into better focus for me (sorry - couldn't resist).

I clearly need to pay more attention to my exact focus point when shooting WA. For example, for this photo, I should have focused on a point nearer to me (and towards the bottom of the frame), rather than in the center of the frame:

However, another complicating factor is that in uw photography we try to get as CLOSE as possible to WA subjects (e.g., corals, sponges, etc.) when shooting WA in order to 1) reduce the amount of water/distortion between the camera and subject, and 2) get more/better light penetration/reflection ON both the subject and the surrounding background. Since DOF is reduced the closer you are to the focal point, this too seems counter-intuitive (if I'm trying to maximize DOF). I suppose it's just a matter of necessary trade-offs...

Bottom line: what I'm hearing people say here is that wide angle lenses will - in and of themselves - help provide DOF, but that using a wide open aperture (e.g., f 2.8 or f4) doesn't really help one get "better" WA photographs, and is not necessarily a good idea, since getting good DOF (or hyperfocus) requires a somewhat smaller aperture (especially at very close distances). Add to that the tendency of WA zoom lenses to have blurry edges at the wide end of the aperture range, and it really sounds like the best "rule of thumb" (all things being equal) is to use a mid-range aperture (e.g., f8) and just be very cognizant of the focal point.

Hi guys!

Any chance that the dome port's curvature has anything to do with it. My understanding is that the dome acts like a lens that projects the entire "outside world" at a very short distance in front of the dome (depending on the dome curvature, e.g. closer for a 6" dome than an 8" dome). That's why you need a diopter when using lenses behind a dome that't can't focus close enough.

Although in reality objects may be anywhere from touching your lens to infinity, I get the impression that the dome compresses all of this into a much narrower and closer range. If that is the case than you don't need a lot of DOF to get it all in focus, so you could get away with larger apertures. It wouldn't explain why a larger aperture seems to have better DOF unless the wider aperture results in better resolution, but I would expect that you would see diffraction based resolution loss only at apertures well below F8.0.

Bart

Personally, I am not a fan of hyperfocal distance shooting on land. The math rule only ensures your DOF range will be recorded with a minimum circle of confusion. The adjusted distance will be recorded sharply. But the edges of DOF will be less sharp including infinity. Infinity subjects are usually very small because they are far away and won’t come out very detailed as you would set focus to infinity. It depends on the content of the picture, but if the priority is sharp infinity, I would always recommend setting the focus accordingly. I was quiet unsatisfied with many shots I did with hyperfocal distance setting until I realised that some minimum circle of confusion does not ensure tack sharp images. Especially with nowadays digital high resolution cameras.

As Bart said, topside hyperfocal distances don’t work underwater due to the dome's negative focal length. As soon as you enter the water, a dome glass acts like a negative dioptre. With an average dome curvature, the auto focus sets the focus ring to somewhat 0.5m if you lock on something far away (infinity) underwater. So even if you like hyperfocal shooting results topside, it won’t work underwater as long as the dome influence is not included into the calculation. And that is not very easy because housing manufactures would not provide required data.

Just a theoretical example by numbers about the relation between underwater distance and virtual image (lens setting), Sigma 15-30mm@15mm behind the big Sea&Sea dome:

Beside DOF as a result of aperture and focal length, fisheyes produce better results with open aperture behind dome glasses compared to wideangles. Different to wideangles, fisheyes don’t suffer under out of focus image corners in conjunction with dome glasses (curvature of the virtual image). That is the reason why some fisheye shots provide better DOF / sharpness wide open than wideangles do. For example, my Sigma 15mm delivers much better corner sharpness @f2.8 than my Sigma 10-20mm @10mm/f4. Even the shorter focal length of 10mm doesn't compensate this.

Julian

this is only peripherally related, but since i started uw photography in the film days, ive always wondered why they didnt design ports or housings with a clear area so you could see the info on the lens. you could see what your distance was, and you wouldnt have to aways guestimate distance, and could use the info for hyperfocal distance calculations and such

very true. I remember that I didn't touch the camera anymore when out of the water so I could check the distance setting when taking the camera out of the housing and read the value from the last shot :-)

Julian

I believe it's got something to do with avoiding any stray rays of light entering the housing and potentially causing unwanted reflections on the lens...