SLR: Is what you see really what you get?

Markster · Feb 21, 2011

I have a second-hand 28-70mm zoom. I'm wondering if it's really not as sharp as I thought. I can't quite tell if I'm just not focusing too crisply or what.

My canon SLR focuses and meters at "wide open" so maybe it's something that kicks in when the iris closes or maybe I'm just imagining it.

Granted I've only had the lens for a few rolls of film now, but either I'm flubbing the focus (a real possibility) or it's coming out a tad blurred.

Is what you see what you get with regards to sharpness? Seems pretty sharp, crisp, and in-focus in my viewfinder. When I get the print back it lacks a bit. Just a tad soft.

What's the best way to test sharpness? I suppose I could compare similar zoom levels with my prime lenses? I could compare 28mm, 50mm, and the 80mm on my telephoto with the 70mm on this one, but that takes a lot of time and effort.

I'm not sorry I got the lens. It's quite a time-saver. If it's a little soft I will live with it. I paid a lower price for it than I would have if it were new. It's more than paid for itself. It would be good to know, though, if I'm going to take a nice photo such as a landscape or a portrait that this might be the second choice to reach for in my bag.

holmburgers · Feb 21, 2011

28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.

Markster · Feb 21, 2011

Sounds quite do-able. I'll try that for a while.

Paul VanAudenhove · Feb 21, 2011

Have you examined the negatives with a loupe? Or are you judging based on prints? Sorry if that sounds obvious, but the negative is the first place to start examining for mistakes.

JayGannon · Feb 21, 2011

holmburgers said:
28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.

Its actually a bad idea, most still zooms do not hold focus when zooming they have a lot of breathing, there are some noted exceptions (11-16 Tokina, 70-200 Nikon) but most do not hold their focus throughout the zoom range.

holmburgers · Feb 21, 2011

Hmmm, that I didn't know. I do know that Canon recommends this practice in the manual for their 35-70mm f4.

JayGannon · Feb 21, 2011

Suprisingly the older slower zooms are much better in that regard, but most current fast zooms fall very much on the bad side of the field, ask anyone trying to shoot video with stills lenses. Its a nightmare!

lns · Feb 21, 2011

A lens might be less sharp in the corners just because it's a cheaper design; most people can live with that for casual shots. If your lens is not sharp generally, though the focus is good, you have a bad lens that you probably should return.

To test, I'd put the camera on a tripod, focus carefully and take some boring pictures of something like a newspaper page tacked to the wall, at a variety of focal lengths and apertures. Take notes to remember which is which. Look at the film (easier if you use b&w or slide film) and prints. If you want to use color negative film, I'd also take the same shots, at the same focal length and apertures, with a normal lens or two, so you can compare the prints. One roll should be enough to see if you have a bad lens

There are some more advanced instructions on the internet, of course, but this should work as a cheap and dirty test.

-Laura

nick mulder · Feb 21, 2011

Breathing I thought was a change in FOV as you rack focus - I didn't know it applied in the reverse sense... I would have said a 'back focus issue'.

Quite willing to be corrected here

...and as an unhelpful aside, one of the peculiarities of an SLR is that you never actually see what went to film - the film sees it, aside for some top and tail getting here and there of the mirror, you see almost everything but what went to film...

moki · Feb 21, 2011

What you see is what you get, unless the mirror or focussing screen is out of adjustement. If the problem only occurs with this one lens, it's the lense's fault, otherwise you should get your camera checked.

These kit lenses are not the sharpest anyway... you could try taking a few photos of a measuring tape with the aperture wide open. That way, you'll see if the focus is just off (a different number than your set distance is perfectly sharp) or if the sharpness is generally lacking (no number appears perfectly sharp). Try this at different focal lenghts to be sure.

Marcus S · Feb 21, 2011

Try photographing a black circle on a white background at various shutter speeds hand held. You will discover once the images are enlarged, that the circle becomes slightly oval at the slower speeds.
The longer focal length will make this even more pronounced. Try using a tripod if the situation allows, to get the best image quality out of your lenses

Markster · Feb 21, 2011

I see that I'm in for another round of test shots. I think I need a focus test. The tape measure is a good idea (or a yard stick).

I was using a shutter release with a tripod to take some test shots in lower light with Ektar 100 and noticed that it wasn't as crisp as I imagined it should be. It's a slightly older tripod, so it's a bit heavier. Not heavy duty mind you, just old. I thought it might be better off than a lighter flimsier one you might find on sale nowadays. The camera didn't seem to vibrate or move at all.

I guess I need to get some more film!

John Koehrer · Feb 22, 2011

No need for a test chart, but a tripod is a necessity for the test. Lock the camera down on the tripod and focus on an "infinity" target & make an exposure, a second or third exposure if you like at different distance. Then duplicate the test with another of your lenses. Compare the negatives

polyglot · Feb 22, 2011

You do see kind of what you get, but keep in mind that a viewfinder has much, much less resolution than the negative you're shooting. The other thing is that many cameras vignette inside the viewfinder optics (it's mostly a matter of how the viewfinder screen is ground and only a narrow range of incident angles showing up, which means an effectively smaller aperture), which means that you will perceive a deeper DOF in the viewfinder than you achieve on film, especially if shooting wider than f/4. So it might be misfocused yet appear to be sharp due to a combination of the viewfinder being low res and showing more DOF.

If you have a split-prism, that should never be in error.

Never focus then recompose, it will cause focusing errors due to rotation of the focal plane. Don't focus then zoom out unless you know for certain that the lens is parfocal. Most are not.

If you're sharpness-testing, a tripod, release cable, stationary high-contrast subject and MLU are mandatory.

It might just be a cheap, soft lens - even the cheapest crap optics will still look sharp in the finder, which kind of demonstrates that you can't judge final image sharpness from the finder. Even crap lenses should give you sharp results at f/11, so test at a range of apertures to see if there's an improvement.

film_man · Feb 22, 2011

What you see is what you should get but if you have a plain screen then it can be tricky to get critical focus. Some plain screens are better than others for getting focus.

Anyway, you seem to have other lenses. Do you have an issue with the other lenses? If not, then this particular lens is just not that sharp.

markbarendt · Feb 22, 2011

holmburgers said:
28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.

Well DOF depends a lot on subject distance.

I have a Tokina 28-80 2.8 that is truly finicky about focus anywhere near wide open even at the wide end. Tends to end up focusing oddly if I'm not being careful.

It also focuses at different points in the focus ring's rotation at 80 and 28 so zooming to focus doesn't normally help.

It is WYSIWYG though.

benjiboy · Feb 22, 2011

holmburgers said:
28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.

I would agree with this, I find it better with zoom lenses to focus on the maximum focal length then zoom back to required one, because they are less subject to "Focus Shift" this way.

Markster · Feb 22, 2011

The zoom has a macro focus at the 70mm stop. Would you suggest testing the short DOF that the macro has? Or would you think this might be in focus but the normal ranges could still be soft?

Lee L · Feb 22, 2011

There are two types of zoom lens designs. Parfocal zooms are designed to stay in focus at the same distance at a given setting throughout their range of focal lengths. Varifocal zooms need to be adjusted for focus as focal length is changed. Parfocal zooms are often 'close enough' (manufacturer's/designer's call), but not dead on focus as the focal length is changed. Parfocal zooms are more difficult to design and more expensive to produce. Ever look at the prices for cine zoom lenses, which need to be more accurately parfocal?

My impression is that as auto-focus came into wide use, this distinction became of less concern and so attention to varifocal vs. parfocal lenses in marketing and reviewing was dropped, and you don't see it mentioned much anymore, to the point that many who entered photography in recent years are completely unaware of the design differences. The distinction between varifocal and parfocal zooms was an often-discussed design feature before autofocus, especially among news and sports photographers who needed the parfocal feature for speed in use.

Lee

Jeff Kubach · Feb 22, 2011

I've been out of focus most of my life!

Jeff

benjiboy · Feb 22, 2011

Jeff Kubach said:
I've been out of focus most of my life!

Jeff

I sharp blow on the side of the head usually re-collimates the brain Jeff.

Marcus S · Feb 22, 2011

You might be inviting a slight softening of the image if you have a filter on your lens without using a lens hood. For myself, no matter what I photograph, a protective lens hood is always present.

Markster · Feb 22, 2011

No lens filter, no hood either. I've pulled the trigger on a few more rolls of film, to arrive in about a week. Will begin some testing then. I guess I'll take some test shots with shallow DOF then some macro with almost no DOF then some "normal" comparison shots against the prime lenses I have.

I guess the real test will be how well it stacks up to the 50mm f/1.8 that came with the camera.

Jeff Kubach · Feb 23, 2011

benjiboy said:
I sharp blow on the side of the head usually re-collimates the brain Jeff.

!

Jeff

2F/2F · Feb 23, 2011

JayGannon said:
Its actually a bad idea, most still zooms do not hold focus when zooming they have a lot of breathing, there are some noted exceptions (11-16 Tokina, 70-200 Nikon) but most do not hold their focus throughout the zoom range.

Would a lens that does not hold focus when the focal length is changed truly be a zoom lens? I was always told that lenses that do this are technically called "variable focal length lenses," and that "zoom lenses" hold focus.

As for the OP, are you asking whether the viewfinder image is an accurate way to judge the sharpness and other technical qualities of a lens? If so, I would say that it is not. Obviously you can see if something is in focus or not, and see obvious things like barrel distortion, but judging sharpness and other such things would be best done by looking at negatives and prints.

There is always the possibility of misalignment of the focusing screen. (At least so on any camera that has a focusing screen!) When this happens, you get slightly different focus on the film than you do in the viewfinder.

SLR: Is what you see really what you get?

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