What am I missing here... (Warning: Noob Question)

[Austin Greene]

Ok everyone, so this is getting a little frustrating.

Recently, there has been a lot of discussion about how f-stop affects the sharpness of your photo, not just the dof. When I say sharpness, I'm referring to the actual area that is in-focus, and the "quality" of the sharpness therein. I will fully admit, that up until a few days ago, I always thought shooting "wide open" was going to be the best thing to do since A) I typically want my backgrounds to be nice and blurred out, and B) I like shooting things that are moving around, and a lower fstop typically yielded the fastest shutter speeds. But sure enough, after a day of shooting at a max aperture of f5.6, all I was getting were images that weren't as sharp as I'd like in the focus area when I reviewed them in the lcd. It wasn't an issue of my subject being outside the dof, and the camera was mounted on a tripod with a 2 second delay pointed at a non-moving subject, yet the images were just plain "not sharp". So I suppose what I'm asking, is if someone would put in laymen's terms why, when a telephoto lens is either at smallest or largest aperture, does it not produce sharp images? I know I must be missing something incredibly simple, but I've done a couple hours of reading and I still don't understand it.

P.S: I heard a lot of people talking about how f/8 is generally a nice good setting to use in terms of rendering a sharp image. Yet when I try that with this particular telephoto, I do not get half the sharpness that I do at f/16. Gah, I am so confused

P.P.S: I put this in the Canon section because this is all on a T3i Even though I know its not a brand-specific question.

 

[dxqcanada]

Each different lens varies in design ... example of an analysis of a Canon lens by DPReview: Canon EF-S 18-55mm 1:3.5-5.6 IS Lens Review: 3. Test results: Digital Photography Review

 

[cgipson1]

How far away are you from the subject? Are you positive you are absolutely nailing the focus? (try liveview magnified.. manual focus as precisely as possible).

Lenses usually have a sweet spot.. around F8 to F11, where they are sharpest. At F8, if you have adequate DOF and are positive that your focus is dead on... the subject should be sharp. If it isn't and you are having to go to F16 to get more DOF.. then the lens might be back or front focusing, and the additional DOF is compensating for that.

try it again.. shoot a variety of apertures (f1.8 to f11.. say 6 or 8 shots)... tripod.. etc... setup a subject on the ground in the grass.... focus on the subject and make sure that your focus is DEAD ON.... then post the pics. Look at where the Actual Focus is.. is it on the subject.. or in front of, or behind it?

 

[MTVision]

Are you only viewing them on the LCD? Have you looked at them on the computer?

I would think lighting and good exposure plays a role in getting a tack sharp image as well.

Not sure if you mentioned but how far away are you. 5ft and under will still give you a pretty shallow DOF.

 

[dxqcanada]

So I suppose what I'm asking, is if someone would put in laymen's terms why, when a telephoto lens is either at smallest or largest aperture, does it not produce sharp images?

Not the easiest thing to answer, but I will give it a shot.

Optical Aberrations. Optical aberration - Wikipedia, the free encyclopedia
There are some aberrations that can be minimized by stopping down the lens.
Different lenses have their own unique physical properties, so their best IQ aperture is also unique.

 

[Natalie]

Can you post some sample images (+ 100% crops if necessary) so we can see that lack of sharpness you're struggling with? That will help us figure out what's causing it and what you can do to fix it.

 

[Overread]

+1 to Natalie's point, its hard to give answers to a question like this without seeing specific examples and also hearing how you shot them and at what settings. Descriptions of sharpness are sketchy at best. I would also reinforce the point that the specific lens you use has a big effect on this. Some are very sharp wide open, get sharper as they close down and then softer thereafter. Some are softer wide open and sharpen up to usable and then soften as they close down further.

The typical pattern is most are sharpest around f8-f11 and then the sharpness will typically start to decrease again as the aperture reduces in size (this is due to diffraction).

 

[Austin Greene]

Thanks everyone for the excellent feedback! Unfortunately I deleted all of my non-keeper photos from the other day, but I will go out and take some more tomorrow to get to you all. I believe a part of my confusion was coming from reading a statement earlier that said all fstops are "universal" and the same across lenses. In a way, I interpreted this to mean that they all had universal dof's. I.E: I could take the kit lens, set it to 50mm and f5.6, and then take a 75-300 and set it to 200mm at f5.6 and expect to see the same sharpness within the dof. ...Wait, did that even make any sense?

I suppose some of this confusion is coming from my not feeling comfortable on which fstop to pick for different situations. I know that for landscapes I want something big (small aperture), and for closeups/portraits I want something small (larger aperture). However, if I'm standing on an overview wanting to take a landscape photo, I havent the slightest clue why I would stop at f11 or f19, when I could crank it all the way to f36 (provided I can still achieve a good exposure). This goes the same for closeups. I know f8 would give me a somewhat blurred background with good focus on the subject, but why not crank it to f4? These are the things that run through my head, and then somehow I'm even more lost once I see that my photos arent as sharp as they should be lol.

If I could look at a flower, and think to myself "I want the background nice and smoothly blurred, so I want *inset fstop here*" I would be one happy camper. Yet something tells me this only comes with experience, and that which fstop I would use would change from lens to lens.

P.S: Next time I head out, I will try out the telephoto again, but taking photos at different fstops other then f5.6 and see if that yields sharper images. Looking back, I kind of feel silly taking photos all day at f5.6 and never really trying to vary from it simply because I got into this mindset of "I want a blurred background, f5.6 will give me that, I should keep it there."

 

[Overread]

A lot of what your considering is simply down to one word - experimentation.
There are depth of field calculators which will give you the cold hard maths of what a specific focal length, distance and aperture will give you in terms of depth of field. However many photographers work on the basis of experience, of getting an idea of what kind of shot they want and what kind of aperture will be suitable. Furthermore, in the real world, many selections of settings are also limited by lighting and subject.

It all sounds rather complicated to start with, however I assure you the more you shoot and the more you experiment whilst you shoot the quicker you'll start to pick this up. Digital is also fantastic for this as you can shoot and review on the computer without having to spend out on developing the shots (unlike on film where each experiment was a cost).


A few links that might help
Depth of field calculator: Online Depth of Field Calculator
Hyperfocal focusing - Understanding Your Camera’s Hyperfocal Distance -- a very good site and worth reading their articles. Hyperfocal focusing is specifically aimed at landscape work and works by giving you the maximum depth of field for a given aperture and scene.

edit:
A bit on diffraction, which is the softening effect you see as you make the aperture smaller and smaller
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
and a depth of field article
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

 

[TimGilbertson]

f8 - f11 being the sharpest is not necessarily the truth because you'll start to introduce diffraction based on the size sensor you're using. Diffraction is the effect of light bending as it passes the blades of the iris. Ideal aperture for APS-C sensors is around f6.3, for full-frame it's f7.1. Now it may not be the ideal aperture for the lens, so you have to feel that out. Most lenses should be adequately sharp by 7.1 in my books.

 

[pgriz]

Overread, as usual, gives excellent advice. The links that he posted are very helpful and should give you a good insight into your issues.

There are many reasons for picture “softness” or “lack of sharpness”.
1) Focus is off,
2) Camera motion,
3) Subject motion,
4) Inadequate DOF,
5) Too-high ISO contributing to noise,
6) Subject is too close (closer than minimum focusing distance of lens)

To disentangle which cause is contributing to the results, you need to do some systematic testing. That means varying one variable, while keeping the rest constant, and seeing how that one variable is contributing to the end result. If you are interested, I’ll post the protocol.

If the issue is depth-of-field, and the degree of blur of the background, then additional factors come into play: focal length, aperture, subject-to camera distance and background to camera distance.

Finally, do not rely on your LCD image for deciding if the exposure and/or sharpness are adequate. Download the image files (preferably RAW) to your computer and use the supplied software (DPP in Canon’s case) or your favorite editing program to examine the image in detail. Many programs allow you to see two images side-by-side, and this is great for comparing the effect of some variable on your images.

Personally, when I shoot something, I almost always vary the aperture, the degree of exposure, the focus point, the viewpoint, etc. and take anywhere from three to 20 shots or more of a scene or subject. All the shots are kept except for the very obvious duds, and I make the selection of the "keepers" only after I have downloaded them into the computer and viewed them on my large LCD monitor. There are usually tons of detail that gets missed when you're looking at the image on the camera LCD.

 

[jcskeeter]

I'm assuming the lens you used for the photos you speak of was the 75-300mm. If that's the case, keep in mind that this is one of the lower end lenses that canon makes. Some might say "consumer level". Now I'm not saying that you can't get a sharp photo with it but it definitely has something to do. There is a sharpness difference if your using a $200 lens vs. a $1000 lens. (Check out my gear, I don't have $$$$ glass either but I think it's worth mentioning there can be a difference.) Also I'm not sure if anyone else mentioned it. Do you have a UV filter or any other filters on the lens? If you have a $12 UV filter screwed on the end of that puppy, that could be a big part of your problem right there.

Also something I've learned about sharpness is, try printing one of your photos on a decent printer. You may find that it's sharper than you think. A while back I had some 8X10's printed from mpix.com and I thought they were sharp on my computer to begin with and when I got the prints I was blown away by the sharpness and detail that was in the print that I just couldn't see on the monitor.

I know a couple people mentioned not relying on the LCD to check for sharpness, which I concur with. But also if you have a sub-par computer monitor you may not be looking at a really accurate portrayal of your photo on the computer as well. (Just a thought.)

 

[iresq]

Since you are new, I'm going to recommend the book Understanding Exposure by Bryan Petterson. I think you will find it very informative.

 

[PapaMatt]

Just a guess but I think it is just a matter of camera movement, Try a tripod. I shoot at 1.8 and 2.8 a lot of the time and it is sharp. A good lens helps but STEADY the camera will help a great deal.

 

[manaheim]

I would bet you money that your perceived sharpness issues have nothing to do with the aperture you are choosing and much to do with something else.

I say this because generally someone who is inexperienced enough to not understand this topic fully is also not experienced enough to really see the differences in sharpness that are evident when making such choices and prone to making other mistakes that affect it in a far more obvious way... such as too long of a shutter time for a certain focal length or simply missing the focus entirely.

Go take some more shots and post some examples. I'd definitely like to see.