understanding DOF

brianf

New member
Hi,
I,m new to DSLRs and also to this forum, so pardon me for appearing slow off the mark. I read a lot of photo books and there is one issue which I do not understand. I have a D3300 with a 28-80 G series 3.5-5.6 lens. I focus manually. Taking a landscape shot at f5.6 my shutter speed will be about 1/250th sec. at ISO 200 say. If I wind the aperture up to f22 to take advantage of a greater depth of field and get everything nice and. sharp the shutter speed will come down to say 1/60th. But am I really shooting at f22? = or is this beyond the limit of the aperture of the lens, and what I end up with is really f5.6 and a overexposed picture. brianf
 

Horoscope Fish

Senior Member
Hi,
I,m new to DSLRs and also to this forum, so pardon me for appearing slow off the mark. I read a lot of photo books and there is one issue which I do not understand. I have a D3300 with a 28-80 G series 3.5-5.6 lens. I focus manually. Taking a landscape shot at f5.6 my shutter speed will be about 1/250th sec. at ISO 200 say. If I wind the aperture up to f22 to take advantage of a greater depth of field and get everything nice and. sharp the shutter speed will come down to say 1/60th. But am I really shooting at f22? = or is this beyond the limit of the aperture of the lens, and what I end up with is really f5.6 and a overexposed picture.
I'm confused... If you set the aperture of the lens to f/22 why wouldn't you "really" be shooting at f/22?
 

Stoshowicz

Senior Member
Hi,
I,m new to DSLRs and also to this forum, so pardon me for appearing slow off the mark. I read a lot of photo books and there is one issue which I do not understand. I have a D3300 with a 28-80 G series 3.5-5.6 lens. I focus manually. Taking a landscape shot at f5.6 my shutter speed will be about 1/250th sec. at ISO 200 say. If I wind the aperture up to f22 to take advantage of a greater depth of field and get everything nice and. sharp the shutter speed will come down to say 1/60th. But am I really shooting at f22? = or is this beyond the limit of the aperture of the lens, and what I end up with is really f5.6 and a overexposed picture. brianf

I came across a thing that says canon uses a different calculation or something which makes fstop confusing, Nikon should be at f22 when you select f22 ,( Im assuming youre calculating an exposure and are trying to correct for the fstop.. but dont have to with the Nikon) But if youre shooting a landscape at say 28mm your hyperfocal distance may be down around 11.5' at f11 , so theres a good chance you just dont have to use such small aperture anyway ,right? so it depends on your subject distance. (Or artistic taste).
Then again, I dont shoot much landscape and so Im curious what someone else has to say.
 
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WayneF

Senior Member
If you shoot at f/22, it will be f/22, and it won't be f/5.6, and if it is overexposed, this will not be the reason. :) The lens works normally at its marked aperture. It may not do f/3.5 in every case, but it will do what the viewfinder says it does. This part is a needless concern. It will be f/22.

No clue what you mean about Canons, but f/stop is an extremely well known characteristic of lenses. :)

Hyperfocal distance is such combination that focusing at this distance will be "sharp" (within the defined limits) from infinity to a close distance. The camera is of course only focused at the one distance, so some approximations apply. There are many possible combinations of hyperfocal, and DOF calculators typically show it (example Online Depth of Field Calculator ) and it depends on sensor size, but maybe a 45mm lens at f/11 focused at hyperfocal 30 feet is said to cover from 15 feet to infinity feet (one example).

If you have a subject at say 20 feet, it will come out sharper if you focus on it at 20 feet instead of the hyperfocal 30 feet. But that loses the effect at infinity. So we do what is important in the situation.

f/22 does cause diffraction, which is a little less sharp, which is a negative, however often the added DOF helps more than the diffraction hurts. Most things are tradeoffs like this. We do what helps.
 
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Stoshowicz

Senior Member
If you shoot at f/22, it will be f/22, and it won't be f/5.6, and if it is overexposed, this will not be the reason. :) The lens works normally at its marked aperture. It may not do f/3.5 in every case, but it will do what the viewfinder says it does. This part is a needless concern. It will be f/22.

No clue what you mean about Canons, but f/stop is an extremely well known characteristic of lenses. :)
.

Oh I just came across a thing that said something about Canon calculating in a bellows factor so that the aperture is open to the level of Illumination one expects without the falloff in light due to that bellows factor with changing focus distance ,, meaning the aperture may not be at the diameter you think it is, in the lens itself.So For Nikons the exposure works out as if the Fstop was correct , and for Canon the aperture size works out to be technically correct for calculating exposures of large format and macro ,,, Or so It read that way. If you havent heard of this , well,., it was the first time I did either.


Large Format Photography: Bellows extension exposure compensation
 
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WayneF

Senior Member
I have heard of the Canon Auto Bellows, but never seen one. The Nikon macro lenses do that too, automatically compensate to show the correct f/stop number for the extension. I suspect your 28-80G does that too? (I've never seen one).

There is a mir page at NIkon Autofocus (AF) Zoom Nikkor 28-80mm f/3.5~5.6D lens group - Index Page on the earlier D lens, which just mentions the G. It was an inexpensive lens, no distance scale, plastic mount and plastic filter threads.

Ken Rockwell has a page (he's usually good on specs), Nikon 28-80mm f/3.3-5.6G

Both say it focuses a little closer than most lenses, about 14 inches, to 1:3.5 magnification (not nearly 1:1, but closer than most lenses). Rockwell says minimum aperture is f/22 - f/38. He does not say if that f/38 is reported, or just actual (but probably is reported at this date You'd know what it does).

The usual nomenclature for a f/3.5-5.6 lens is that maximum is 3.5 at wide angle zoom, and f/5.6 at maximum zoom. Several lenses do this. This one also focusing closer has another possibility, of a smaller aperture up close (both wide and tele).

You could determine what it does...

With a subject more distant, say 20 feet (doesn't really matter, but not close)
At maximum aperture (f/3.5), zoom to widest angle and focus and note reported aperture.
Should say 3.5.
Also at maximum aperture, zoom in to longest telephoto and focus and note reported aperture.
Should say 5.6.

Then repeat at minimum aperture, which should say f/22 wide, but I don't know tele. I think most today would say f/22 tele too. The problem isn't that they cannot stop down, but they cannot open wider than maximum.

Then at closest distance (14 inches), repeat both wide and telephoto.
At closest distance, my guess is both wide and tele report a little larger number, due to the closeup magnification.
Or it seems possible that it might report some combination of the two effects.

But I'd guess that in any situation, the lens is doing what it reports it is doing.



 
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