Cropping into a 85mm f1.8 = poor man 190mm f1.8?

Earwax69 wrote in post #14888722
Thanks wilt, i think you covered it all. It also why fullframe camera make sharper pictures, because they use more of the lens resolution.

That is too simplistic an answer. There are a few factors that go into the sharpness of an image. You can get a razor sharp image on a crop body that would rival that of a FF, provided proper glass for the APS-C. Glass, the strength of the AA filter, ISO performance, contrast, viewing distance, enlargement factor, etc all go into the perception of sharpness.

With the T3i, if you cannot get sharp images, it will most likely be one of the following:

- Lens quality (either just plain ol' optics issues, or not in the sweet spot of the lens, either FL or aperture)
- Lens front/back focus or missed focus
- Improper settings, like shutter not fast enough/motion blur by either shooter or subject material
- DOF too thin and wasn't enough to cover the intended target (or movement right before the shot)
- Too high an ISO and/or too low an exposure
- No or improper post processing
- Using the in-camera JPG vs using Raw

A digital image can be 'sharp' (or not) AND it can also be 'high resolution' (or not).

An image can be 'sharp' without it being also 'high resolution'. And an image can be 'high resolution' yet not be 'sharp'.

A larger image requires less magnification to achieve a certain final size, which can affect both the perception of sharpness and resolution.

And I can change the perceived sharpness of an image (with USM or other sharpening algorithms) without a change to the fundamental resolution of the image.

'Digital zoom' will never equal the IQ achieved with true optical zooming (FL change)

170mm would mean you are left with one fourth the resolution. So that's 4.5 megapixels. you will also be magnifying any abberations and noise.

4.5mp... Ouch! Somehow it can do for black ant white picts where the grain add to the look but for color pictures, it is stretching a lot.

Another question: ive read thAt the 200mm f2.8 let more light inside than the 70-200 f2.8. ThAt mean faster shutter speed? How can thAt be if both are 2.8??

Yeah, you get a big drop. A doubling of focal length means cutting the diagonal angle of view in half. Half the diagonal means 1/4 the area. 1/3 the diagonal means 1/9 the area. 1/4 means 1/16 the area, etc, etc. 1.4x gives you half the resolution, so that's about as far as I'd want to plan on going. That turns 85mm into 120mm.

They both have an aperture of f/2.8, but if a lens has vignetting or lower light transmission due to optical formula, it can actually let in a lower enough amount of light to cause a slight difference in exposure. I don't know if the 70-200mm fits that description or not, but zooms are at a disadvantage due to complexity of optical design.

Incedentally, this is why movie lenses are rated in T stops, which rates actual transmission rather than just aperture size.

Then there's no real advantage of owning the 200mm prime over the zoom except for the price, size and weight I believe. Those are not small advantages though. Weight do matters.

Earwax69 wrote in post #14889910
Then there's no real advantage of owning the 200mm prime over the zoom except for the price, size and weight I believe. Those are not small advantages though. Weight do matters.

There's no advantage except for all those advantages?

size difference is pretty huge: http://media.the-digital-picture.com …ephoto-L-Prime-Lenses.jpg

If you want the zoom and price is a factor, consider the Sigma 70-200 f2.8 HSM macro. Those are about the same price as the Canon prime.

I wish they'd make a 70-200 f2.8 in plastic for aps size cam like the 17-55.

Earwax69 wrote in post #14892799
I wish they'd make a 70-200 f2.8 in plastic for aps size cam like the 17-55.

It would still be a 70-200 2.8. Just buy the L, or look at one of the Tamron/Sigma/Tokina 70-200s.