Optimum Aperture Question

[rickp1]

Hey guy and gals,
I'm trying to find out what the optimal f stop is for my Canon 50D lenses. I have a 70-200 f2.8 and a 17-55 f2.8 and a 85mm 1.8 prime.

I called canon and they said between f/stop 8 and 11. But I would still like to know if anyone had a different opinion on that.

Thanks
R.

[Arstine]

That's a bit of a broad question. I use my aperture setting depending on the depth of field I want so there is no optimum in general. It's dependent on what I am looking for in each shot.

Or are you asking for best sharpness? If that's the case it's up to the lens.

[ddr]

perhaps by optimum you mean the highest level/amount of sharpness across the frame at a particular aperture

u need to look at the MTF charts for a given lens on a site like photozone.de

usually, it's about a stop or two down from the largest aperture

[rickp1]

Hey guys,
Actually optimum sharpness was what I meant. I called Canon and they said f/stop 8 to 11 for their lenses.

Thanks
R.

[Elmo]

Several things come into play. Design criteria generally tend toward f8-f11 But each individual lens of a type will vary due to mfg tolerences. Also this will depend upon whether you are shooting film or digital. To complicate things mare different sensors (pixel density) will cause differences.

Just as a hint most lenses when used on digital cameras start to fall off after about f8 .

[rickp1]

Hey Elmo,
Actually that's kind of what canon said too. Question, what do you mean start to fall off after f/8. Do you mean start to lose sharpness?

R.

[ddr]

look at the charts

[Elmo]

Originally Posted by rickp1

Hey Elmo,
Actually that's kind of what canon said too. Question, what do you mean start to fall off after f/8. Do you mean start to lose sharpness?

R.

Yes it will loose some sharpness (little but is noticeable in larrger prints). It is because of defraction and interaction between the lens and the sensor. Two issues come into play one being the reflectiveness of the sensor (More than film) the second is with sensors there is a fixed pixel size, with film the exposed area varies with the area of light being projected on the film.

[rickp1]

ddr,
I'm not a pro and I don't know what I'm looking at.

R.

[ddr]

this is just the test of one website, so don't read it like the gospel. but it's better than nothing and it makes sense to me when I refer to the lenses that I own or owned.

for example, this shows sharpness at various focal lengths and at various apetures:


it's just a graph for reference since you asked for it. i don't think u need to b a pro to understand it. did u try to look at the website i linked to?