Sharpest f stop?

Big Mike · Sep 26, 2007

So basically Fstop is Aperture? And Aperture is the amount of light let into a lens when taking a picture?

The F-stop or F number is a ratio between the focal length and the diameter of the aperture opening. F4 is actually 1/4 which means that the aperture's diameter is one quarter of the focal length.

'Aperture' is not the amount of light coming into the lens...but a larger aperture lets is more light. Smaller F numbers represent larger apertures.

So basically higher Aperture settings make for a sharper, more clear image?

That might be a simplistic way of looking at it...but for the most part, yes. The higher (smaller) the aperture...the more DOF you have, so more things may be in focus. However, as people are pointing out...each lens seems to have a 'sweet spot' which is sharpest. I have always heard and found that most lenses are best from...one or two stops from wide open...to F8 or F11.

Garbz · Sep 26, 2007

abraxas that's not going to happen. If anything the ISO will keep going up as it becomes harder to make sensors with high ISO and low noise insensitive to light. Look at the D200 (ISO100) D300 and D3 (ISO200). To go slow you'll have to get ND filters. They may even come standard in the lens of the future :lol:

six-five-two: http://www.photozone.de/8Reviews/lenses/pentax_1855_3556/index.htm that's your 18-55. Actually not all that sharp anywhere really but the lens is very consistent. Slight but probably not notable edge at f8.

skieur: Out of all the f/2.8 from minolta and tokina lenses none of them are perfectly diffraction limited (i.e. sharpest at f/2.8). They all follow the pretty normal bell curve and are sharpest at around f/5.6 (mostly 2 stops from open):
http://www.photozone.de/8Reviews/lenses/tokina_1650_28_nikon/index.htm
http://www.photozone.de/8Reviews/lenses/tokina_50135_28_nikon/index.htm
http://www.photozone.de/8Reviews/lenses/tokina_100_28/index.htm
http://www.photozone.de/8Reviews/lenses/minolta_50_17/index.htm
http://www.photozone.de/8Reviews/lenses/minolta_85_14/index.htm

kundalini · Sep 26, 2007

Nice link Garbz. Cheers.

The_Traveler · Sep 26, 2007

Can anyone point me to a tutorial on reading/understanding these kinds of sharpness charts?

Helen B · Sep 26, 2007

You could start at Norman Koren's site (look for the 'Understanding MTF' page, among others) and the affiliated site dedicated to Imatest, his software that is used for the Photozone tests. I have a little experience with Imatest, so might be able to answer some questions.

One thing to remember is that these results are for the plane of perfect focus. They do not represent the sharpness of objects that are in perfect focus away from the film plane - ie they do not tell you how sharp the image of something will be when it is not in perfect focus. In most photographs of three-dimensional items very few parts will be in perfect focus.

Best,
Helen

xfloggingkylex · Sep 26, 2007

Great link Garbz. I had no idea that diffraction can start as early as f/8. Here I had been thinking my 50 1.4 was sharpest at f/8 but in reality thats where diffraction starts, and 5.6 is the sweet spot.

The_Traveler · Sep 26, 2007

Helen B said:
You could start at Norman Koren's site (look for the 'Understanding MTF' page, among others) and the affiliated site dedicated to Imatest, his software that is used for the Photozone tests. I have a little experience with Imatest, so might be able to answer some questions.

Best,
Helen

Thank you for excellent refs.
I looked at your "snaps' and especially liked http://www.usefilm.com/image/1178410.html

abraxas · Sep 26, 2007

Garbz said:
abraxas that's not going to happen. If anything the ISO will keep going up as it becomes harder to make sensors with high ISO and low noise insensitive to light. Look at the D200 (ISO100) D300 and D3 (ISO200). To go slow you'll have to get ND filters. They may even come standard in the lens of the future ...

I sort of thought so... As long as I don't have to change too much- Or learn stuff. Maybe just keep it simple, things I can ask instead of lookup.

The future sounds complex.

six-five-two · Sep 26, 2007

Garbz, is there any other 18-55mm Pentax lenses that will fit on my K100D that is sharpest then the kit lens? Because I think there is only one type of 18-55 made by Pentax for the K100D mount thingy...

six-five-two · Sep 26, 2007

I also did a small experiment, went to some website with pictures taken by the Pentax K100D. Wrote down 33 aperture settings then averaged it.. the average is 8. So apertures 8-11 should be good.

xfloggingkylex · Sep 27, 2007

six-five-two said:
Garbz, is there any other 18-55mm Pentax lenses that will fit on my K100D that is sharpest then the kit lens? Because I think there is only one type of 18-55 made by Pentax for the K100D mount thingy...

check out third party lenses such as sigma. They have some excellent 18-50 2.8 lenses, and IM sure they will have something around the kit level.

six-five-two · Sep 27, 2007

Well I found a Pentax 18-50mm lens. I saw pictures and they were fantastic... then I looked at the price and my jaw dropped... $900 for that lens. Damn!

Searched for some Sigma lenses. The cheapest one was $180 and was worse quality then my kit lens. I guess good lenses are about $600-900... you get what you pay for. Thanks for all your help though.

xfloggingkylex · Sep 27, 2007

sigmas 18-50 2.8 is like 400.

Iron Flatline · Sep 28, 2007

abraxas said:
I'm waiting, and this could be forever, for a camera, hopefully Nikon that has ISO of 50 or even 25. This would be cool to me. Looks like it's going the other way though.

The Canon 5D (and probably several other high-end Canon cameras) have ISO 50.

jstuedle · Sep 28, 2007

abraxas said:
I learned this twice- once in high school many years ago and again about 10 years ago when I first read about the f/64 club.

I'm waiting, and this could be forever, for a camera, hopefully Nikon that has ISO of 50 or even 25. This would be cool to me. Looks like it's going the other way though.

As sensors get more advanced, their native ISO, the minimum ISO to fully saturate the individual pixel without amplification, will only go up. Early sensors needed more light to expose the chip. As we get sensors with lower noise levels, the higher the minimum ISO will be. This is a very good thing. We are seeing image quality going up while minimum ISO is going up as well. I do long for good glass with smaller minimum apertures. I would love to see all prime lenses to have at least a min. aperture of f/32 or f/45. And it would not bother me to see my macro lenses have a min. aperture of f/64. Just another thing on my wish list.