How much are you dying to get your hands on a 5D Mark II?

[Sean Setters]

Well, when considering the move to the 5D Mark II and selling the 17-55mm, I was considering the 24-105mm f/4 L IS, the 17-40mm f/4 L, or the 24-70mm f/2.8 L as a replacement. Because of the amount of money involved in upgrading cameras, I was probably going to have to start out with the 17-40mm.


I've really become accustomed to having wide apertures and image stabilization available, and it's difficult for me to let go of either one. On Saturday I'll be shooting a wedding, and I'll probably be glad to have both. Oh, and props to Bryan on the review of the Bogen 640B monopod. I got mine a few days ago and tried it out last night. It's a very solid piece of equipment, and doesn't seem to feel as heavy as it looks. And the Bogen 234 Swivel Tilt monopod head holds up my 50D, battery grip, and 70-200mm f2.8 L IS lens quite well. I plan on using the monopod a little bit during the reception when I can't use the flash.

[Daniel Browning]

I've really become accustomed to having wide apertures and image stabilization available, and it's difficult for me to let go of either one. On Saturday I'll be shooting a wedding, and I'll probably be glad to have both.

If that was your concern, then you can rest easy. The 24-105 has a *wider* aperture than the 17-55 at every equivalent field of view and perspective. For example, the aperture on the 17mm at f/2.8 is 6mm. The L at 27mm f/4 is 7mm: 17% wider aperture. The larger front element of the L points toward this fact too.

The focal ratio (not aperture) is one stop narrower (f/4 vs f/2.8), so the light intensity per area is twice as dim. However, the total amount of light is more than double, thanks to the much larger area and wider aperture. So you can up the ISO or use -1 EC to get the same shutter speed as with f/2.8 and still collect a half-stop more light in total. The resulting image will be superior to the 17mm on the 30D, and only slightly better than on the 50D.

I hope that help.

[Sean Setters]

Thanks for the additional info. However, I'm a little bit confused. The point of having wide apertures (for me, at least) is enabling a small depth-of-field, lowing the ISO needed, or permitting to use faster shutter speeds. If I take a picture using the 17-55mm on a 50D, at 20mm and f/2.8, won't I be letting in twice the amount of light than if I take the same picture with the 24-105mm, at 32mm and f/4 (at these settings, the 1.6 multiplier means the subject is framed exactly the same for both shots)? Are you saying that because I'm using a 1.6 crop sensor, the equivalent aperture on the 50D would be narrower on a full-frame sensor? You also said that the front element of the 24-105mm is larger, but I'm not quite sure what you mean and how that's relevant? I'm really lost on this one. Do you have a site that I can check out that maybe goes a little more in-depth?





17-55mm f/2.8


Filter Size 77mm


Groups/Elements 12/19


Length 4.4" (110mm)


Max Diameter 3.3" (84mm)





24-105mm f/4


Filter Size 77mm


Groups/Elements 13/18


Length 4.2" (110mm)


Max Diameter 3.3" (84mm)

[Daniel Browning]

Thanks for the additional info.

It's my pleasure. One day I learn something from you, the next it's vice-versa, sharing knowledge is fun.

The point of having wide apertures (for me, at least) is enabling a small depth-of-field, lowing the ISO needed, or permitting to use faster shutter speeds.

Agreed.

If I take a picture using the 17-55mm on a 50D, at 20mm and f/2.8, won't I be letting in twice the amount of light than if I take the same picture with the 24-105mm, at 32mm and f/4 (at these settings, the 1.6 multiplier means the subject is framed exactly the same for both shots)?

Kind of. Yes, at f/2.8 you will be letting in twice the amount of light *per area*, but the total amount of light captured will be much smaller. For example, the lens on a very tiny 1/1.7" Digicam with a 7mm f/2.8 lens also has the same perspective/FOV as your 20mm, and it also captures the same amount of light per area, but the total amount of light falling on the sensor is much less. In fact, f/2.8 on a digicam is the same amount of light as f/14 on your 50D! (And f/19 on the 5D2.) It's no wonder digicams struggle in low light. The reason is sensor area.

In the same way, a full-frame f/4 lens focuses more light than an f/2.8 APS-C lens: it's just spread out over a wider area. So the full-frame camera has about a 1/3 stop advantage in low light at f/4: you could upgrade to the 5D2 and 24-105 f/4 and still get all the same DOF, noise level, and shutter speed. In fact, it's more than a 1/3rd stop better at f/4.

Of course, if you take it even further, and go with f/2.8 on the 5D2, then you'll have a 1.4 stops thinner DOF, and that much more low light power.

Are you saying that because I'm using a 1.6 crop sensor, the equivalent aperture on the 50D would be narrower on a full-frame sensor?

Yes. To get the same depth of field on a full frame sensor, one must stop down approximately 1.4 stops. Since f/4 is only 1 stop, shooting the 24-105 wide open will give you less depth of field (and more total light).

You also said that the front element of the 24-105mm is larger, but I'm not quite sure what you mean and how that's relevant?

I shouldn't have mentioned it: the front element just confuses things. But since I did, I should explain. The front element is the entrance pupil, i.e. the big piece of glass that your brides see on the front of your lens. Generally, the aperture (not focal ratio) determines the size of the front element. A 400mm f/5.6 has an aperture of 71mm (400/5.6=71), just like the 80mm f/1.2 (80/1.2=70), so the front element of both lenses will be a certain size. Sometimes they are larger for higher quality (less vignetting) or other optical design reasons. If you compare the front elements of the 17-55 and 24-105, you'll see that the L has a bigger one. Generally, this means that it is longer, faster, or has a bigger image circle. In the case of the L compared to the 17-55, it is longer, slower, and a bigger image circle. Clear as mud?

I'm really lost on this one. Do you have a site that I can check out that maybe goes a little more in-depth?

Yes! I'm glad you asked. There is one web page that explains all of this, and more, in excrutiating detail.

http://www.josephjamesphotography.com/equivalence/

But I would be happy to answer any additional questions about this topic.

--
Daniel

[Sean Setters]

Thank you very much, Daniel. I fully understand what you're saying now. I won't hesitate to go to the 24-105 f/4 L IS when I finally upgrade to the 5DM2...besides, if I really need faster glass I can always use my 70-200mm f/2.8 IS (and the lens won't be as long as it is now, so it should be a little more useful for everyday shooting).



Again, thank you for your patience. Sometimes it takes some effort to bore through my thick skull. ;-)

[Daniel Browning]

You're welcome!