Optimal focal lenght on a zoom lens?

Question

ok im still on this photography and lens sharing info forum buzz thing so please all contribrute

so a zoom lens not a prime, lets say a 70-200mm, at what focal lenght will the lens be sharpest?

70mm / 200mm or 135mm?

if you know give a reason, if you dont cool give a number and a theory or just a number

My thanks is 2 points to all + one luck person will be chosen, if its a tie then best theory/explaination gets the big ten.

Ive been told the answer but it could be wrong?its got to do with the distance between the elements in the lens.

Evan you are a genius, wow thanks so much you are so valuable and your answers are (dont know the words)

great is a little understated

thanks

Answer 1

There is no set answer to this.

Resolution is commonly measured in MTF (Modulation Transfer Function). A lens's peak MTF values will vary between different aperture settings, and these values may peak at different focal lengths in a zoom lens.

For the purposes of this answer we will use the simplified Imatest results from www.photozone.de, which provides independent testing of SLR lenses. It is worthy of note that this testing uses APS-C sensors, so we will only talk about center-resolution for now. If we included border or extreme border, it will only serve to increase the variability of the data and further illustrate my point.

Example: Using the Canon EF 70-200 f/2.8L (non-IS), we can see that when shooting wide open the lens will achieve peak MTF at the short end (70mm) with a value of 1837 LW/PH. However, the peak MTF for the lens overall is at f/5.6 at 135mm, with a value of 1999.5 LW/PH.

Now, if we compare this lens's little brother the f/4L (non-is), we will find that overall peak MTF is at 70mm f/5.6. Interestingly, this lens drops slightly in MTF at the mid range at 5.6 and then picks up again at the long end. This lens also peaks in sharpness at the short end when shooting wide open.

Despite this variability, there are a few general guides:
Zooms are typically (but not always) softest at their long ends. Ultra wide angle zooms are often softest at their widest extreme. "Standard" zooms usually perform best in MTF around the 50mm mark. Of course, lenses tend to peak in sharpness in mid-aperture (often around f/5.6) ranges and then diffraction causes these values to begin to fall off somewhere around f/8 or higher.

So to answer your example, I can say that it is very likely that your hypothetical 70-200mm lens WON'T be sharpest at 200mm at any aperture. Whether it peaks at the short end or in the middle will depend on what the lens-design wizards decided to prioritize in that particular model. It will probably hit peak MTF around f/5.6-8. The peak sharpness for wide-open aperture shooting may differ from the overall peak MTF, as well.

Answer 2

Again, this varies heavily on the manufacturer of the lens and its speed. A 70-200mm f/2.8, say a nikkor for the sake of arguement, is pretty sharp througout the entire focal range. It's also top end glass, costing around $1600 so in this case you get what you pay for. I have read that the sigma 70-200mm f/2.8 is a little soft at 200mm @ f/2.8 than the nikkor. And I am also guessing it has to do with the different lens elements. That's why primes are always better, IMO.

EDIT:

The answer below me is a way better explaination than I could give, good job Evan!