DOF and zoom lenses ???

[explody pup]

This has been mentioned a couple of time to me and I still can't really get my head wraped around the "why" of it...

So, when you get a zoom lens (say a 35-80mm for example) you aren't going to have as good of a DOF as you would with a fixed-focal length lens unless you're willing to shell out some serious dough for a high quality piece. Correct? If so, then why?

If you can't tell, I'm still shopping around for a new lens.

Thanks.

 

[Big Mike]

The DOF has to do with the aperture of the lens. The wider (smaller F number) that the lens can open to, the more shallow you can make the DOF.

This would be a benefit for portrait photography where you want a shallow DOF to make the background blurry.

For something like landscape photography, usually a large DOF is better. For this you want a small aperture (larger F number). Just about every lens will stop down enough to get plenty DOF.

The cost factor when buying a lens is how wide can it open (max aperture, smaller number).

Besides a shallower DOF, a wider aperture will give you the ability to shoot hand held in darker situations. Because a bigger aperture lets more light in...the shutter can be faster (hopefully fast enough so that camera shake is not noticeable.)

Check the personal web page forum here. There is a link to a site called interactive camera or something. It is very helpful to show you how shutter speed, aperture and focus work on a camera.

 

[explody pup]

Okay, cool. I guess I shouldn't worry so much then. The lenses I'm looking at have are f/4.0-5.6 (35-80mm) and f/4.7-5.6 (80-200mm) which I guess isn't too bad. If I need a really shallow DOF I can always use my stock 50mm.

Thanks, Big Mike.

 

[Big Mike]

That's pretty much what a lot of us do. It's nice to have a general zoom when you need it but a fast prime lens like 50 F1.8 or F1.4 are a life saver when you need a fast lens. Plus they are dirt cheap compared to other lenses.

Another thing to note about the price of zoom lenses is the quality of glass & coatings etc. The cheap consumer lenses that come with entry level SLR cameras these days are at the bottom of the scale when it comes to quality build and quality glass. The expensive pro lenses use the best technology and the best glass to get the durability and quality that the pros require. For an amateur, it's not really much of an issue.

The prime lenses usually have pretty good glass and can rival the pro zoom for picture quality.

 

[tr0gd0o0r]

If you're really looking for a shallow depth of field, another thing to think about is the focal length of the lens. The shorter the lens the less depth of field you get.

 

[voodoocat]

If you're really looking for a shallow depth of field, another thing to think about is the focal length of the lens. The shorter the lens the less depth of field you get.

Well longer focal length lenses will give you a shorter DOF. The longer focal lengths allow you to really seperate a subject from it's background.

 

[tr0gd0o0r]

oops, had that a little backwards. Sorry for hte misinformation and thanks for the quick correction

 

[voodoocat]

oops, had that a little backwards. Sorry for hte misinformation and thanks for the quick correction

You know me tr0g. I'm always here... and always ready to correct moments of dyslexia.

 

[ksmattfish]

So, when you get a zoom lens (say a 35-80mm for example) you aren't going to have as good of a DOF as you would with a fixed-focal length lens unless you're willing to shell out some serious dough for a high quality piece. Correct?

If so, then why?

f/# = focal length divided by aperture size

so f/4 on a 50mm lens means an opening of 12.5mm.

f/2 means an opening of 25mm.

And f/1 would mean an opening of 50mm.

Basically, the design size and the glass are doubling for each stop. More glass = more expense. And zooms already have a lot of glass. To keep the price down, they design consumer zooms that go down to around f/4. This keeps them affordable, and they'll still handle most photography; f/4 zoomed out part 100mm starts to get pretty shallow. But someone is always willing to pay for the extra stops, so the "pro" gear usually goes down to around f/2.

Prime lenses are simpler design, and it's easier to make economical fast lenses.