[Nesster]

2/3 stops in shutter speed:

1/15 -> 1/20
1/30 -> 1/50
1/60 -> 1/100

That is, if you were shooting at 1/30 with the 1.8 lens, you could shoot at 1/50 at f/1.4

In ASA speed terms, 200 to 400 is 1 stop, 400 to 800 is also 1 stop. 400 to 640 is 2/3 stops.

As discussed above, the shallow DOF will definitely create 'interesting' photographs, while you probably would be more interested in a deeper DOF for a set of party shots.

What a faster aperture does for you, however, is let through more light when focusing wide open. So even if you were shooting at f/2.8, say, all else being equal the 1.4 lens will let you see and focus a bit better.

[Sean Nelson]

Certainly didn't mean to discount Nesster (or anyone else's response). I'm not at the level where I understand the "2/3 of a stop" impact though.

It's all about exposure. If you have a lens that is 2/3 of a stop "slower" (or, in other words, dimmer), then you have to use a shutter speed that is 2/3 of a stop longer to compensate. In order words, the shutter will have to be open for 66% longer than it would with the faster lens.

For example, if the f/1.4 lens would give you an exposure of 1/15 of a second, you're going to need an exposure of 1/10 of a second with the f/1.8 lens to compensate. In practical terms, anything moving in your picture (and in fact the entire picture itself if you move the camera more than the Shake Reduction [SR] system can compensate for) will have 66% more blur due to the longer exposure.

[Sean Nelson]

What a faster aperture does for you, however, is let through more light when focusing wide open. So even if you were shooting at f/2.8, say, all else being equal the 1.4 lens will let you see and focus a bit better.

Actually this isn't true. The microprism focusing screens in modern cameras only direct a light cone equivalent to around f/2 to f/2.8 toward your eye - anything else is lost. You can see this for yourself by mounting an f/1.4 lens on your camera, setting the aperture to f/2, and hitting the depth of field preview button. You won't see any dimming in the viewfinder - you'll have to go to f/2.8 or more before you can see a difference.

[felix68]

2/3 stops in shutter speed:

1/15 -> 1/20
1/30 -> 1/50
1/60 -> 1/100

That is, if you were shooting at 1/30 with the 1.8 lens, you could shoot at 1/50 at f/1.4

In ASA speed terms, 200 to 400 is 1 stop, 400 to 800 is also 1 stop. 400 to 640 is 2/3 stops.

As discussed above, the shallow DOF will definitely create 'interesting' photographs, while you probably would be more interested in a deeper DOF for a set of party shots.

What a faster aperture does for you, however, is let through more light when focusing wide open. So even if you were shooting at f/2.8, say, all else being equal the 1.4 lens will let you see and focus a bit better.

Isn't it funny that the shutter speed gained by f1.4 over f1.8 is exactly the one over focal length rule of thumb for the 50mm and 31mm? Not taking motion stopping ability into consideration, the 31mm should be exactly as easy to hand hold at f1.8 as the 50mm is at f1.4. Personally, I will take the 31mm at f1.8 over the 50mm at 1.4, in terms of image quality. With the 31, I don't even think about taking it down to f1.8. With the 50, I would rather shoot at f2.0 if possible.

[Alfisti]

Thanks for all of the feedback everyone. It's not the bokeh that I'm wondering about, nor about the difference between 31mm and 50mm. I'm mainly wondering if the 1.4 to 1.8difference is significant in low light, non-flash scenarios?
thx

No it's not.

[lbam]

This discussion has been quite good for me as I like to try and get shot's in bars with fast lenses...(no flash), it proves there are no silly questions or that there's always stuff to learn.
I'd love to try the 31mm (if I had one), over the 58 nokton, or A50 f1.7.
The DA40mm might be reasonable as it focuses so fast also (despite it's small max aperture).

[lbam]

A larger aperture than F2.0 might not be better for the eye, but would it be better for autofocus if you were using an autofocus lens? I'd imagine so..

---

Last edited by lbam; 10-21-2008 at 11:42 AM.

[morfic]

There is always the option of getting 30mm and f/1.4 in form of a Sigma 30/1.4, if during birthday parties most your subjects are in the center of the frame it should be a nice lens to have (its weakness are extreme corners), and cost half the 31/1.8 price. With HSM it would also be quiet and quick.
And at 30mm you can take advantage of the extra speed over the 31/1.8

That said, nothing beats a flash and a nice f/4 or f/5.6 aperture where most lenses peak and you have enough DoF for people to not move out of the focal plane while you take the shot.
Get one with a built in bounce card or have an index card on you, a nice sparkle in the eyes will bring a little pop to the otherwise rather flat bounce flash pictures and lighten up some of the shadows under eyes/chin if you bounced too close to them.
Remember you want to bounce to where you want the light source to be, not "halfway between the subject and you" as some like to say. a nice (white) wall behind or next to you makes a perfect candidate.

[Wulifou]

That said, nothing beats a flash and a nice f/4 or f/5.6 aperture where most lenses peak and you have enough DoF for people to not move out of the focal plane while you take the shot.
Get one with a built in bounce card or have an index card on you, a nice sparkle in the eyes will bring a little pop to the otherwise rather flat bounce flash pictures and lighten up some of the shadows under eyes/chin if you bounced too close to them.
Remember you want to bounce to where you want the light source to be, not "halfway between the subject and you" as some like to say. a nice (white) wall behind or next to you makes a perfect candidate.

morfic,
even if this doesn't really belong to the original post anymore, I hope you don't mind if I take your post as an opportunity to ask some questions about flash uses. I mainly use the 50mm F1.4 because all my shots with flash produced so far results I didn't like. Since you seem to know the topic, is there
a) a particular flash you can recommend to a beginner (K20D)
b) some site/tutorial on "how to use a flash without producing crap" ? (preferably with pictures illustrating the effects direct flash/bounced flash/ diffused flash ....) I suspect I'm pretty close to stupid, but I haven't found one explanation so far, that helped me producing better pictures.

[thePiRaTE!!]

It's all about exposure. If you have a lens that is 2/3 of a stop "slower" (or, in other words, dimmer), then you have to use a shutter speed that is 2/3 of a stop longer to compensate. In order words, the shutter will have to be open for 66% longer than it would with the faster lens.

For example, if the f/1.4 lens would give you an exposure of 1/15 of a second, you're going to need an exposure of 1/10 of a second with the f/1.8 lens to compensate. In practical terms, anything moving in your picture (and in fact the entire picture itself if you move the camera more than the Shake Reduction [SR] system can compensate for) will have 66% more blur due to the longer exposure.

The math and everything is correct, but putting it this way makes it feel more dramatic. Not to pick - because you're right of course - but in use, it really doesn't change a lot, shutter speed 1/10 vs 1/15 for example.

I would compare it to saying 16.6 cents has 66 percent more purchasing power than a dime. Its true and dramatic, but largely inconsequential.

Again, no offense meant, I just didn't want the OPs heart to sink after his choice, lol.

[beaumont]

Thanks for all of the feedback. A very knowledgable group indeed. I believe the general consensus is (assuming I don't need the 50mm focal length) that I may have been correct in feeling that I was "covered" with the 1.8 of the 31ltd as opposed to buying the 50mm just for the 1.4.
Thanks again for the great info. Lot's of learning happening here...

[Sean Nelson]

A larger aperture than F2.0 might not be better for the eye, but would it be better for autofocus if you were using an autofocus lens? I'd imagine so..

The autofocus should get the full benefit of (for example) an f/1.4 light cone since the microprisms in the focusing screen aren't in it's path. It may actually be TOO good - I find the focus on my f/1.4 lens is very twitchy at objects closer than several feet compared to f/2.8 or f/4 lenses.

[Sean Nelson]

In practical terms, anything moving in your picture (and in fact the entire picture itself if you move the camera more than the Shake Reduction [SR] system can compensate for) will have 66% more blur due to the longer exposure.

The math and everything is correct, but putting it this way makes it feel more dramatic. I would compare it to saying 16.6 cents has 66 percent more purchasing power than a dime. Its true and dramatic, but largely inconsequential.

Well... would you rather pay $2.00 or $3.33 for a gallon for gas? It's exactly the same percentage, and more than a bit consequential, IMHO.

Actually Felix68's point about the 31mm being less affected by camera shake is very much to the point. This should make image blur due to camera movement pretty much a wash between the lenses - but image blur due to the subject moving would still be 66% worse (assuming you moved closer to your subject to have it fill the same portion of the frame).