Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Nikon Lenses

Reply
 
Thread Tools Search this Thread
Old Jul 4, 2010, 6:40 AM   #1
Senior Member
 
Join Date: Apr 2004
Posts: 688
Default lens ed

Why is it that a Dx lens being used on a dx camera is called ?mm and is really a ?mm.
example a nikon 35mm lens is really a 52.5mm lens, jump high, jump low it is a dx lens no matter what kind of nikon dx camera it is put on, it will always be a 52.5mm lens, so why not just call it that ?

Dave
T&T
dafiryde is offline   Reply With Quote
Sponsored Links
Old Jul 4, 2010, 7:19 AM   #2
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,572
Default

The focal length of a lens is a physical property of that lens, and doesn't change with the camera it's mounted on. What does change is the size of the image sensor, and a smaller image sensor gives the same lens a narrower angle of view. The 'Crop Factor' doesn't actually change the focal length, just the angle of view, such that a 35mm lens on an APS-C body ends up having the same angle of view as a 52.5mm lens (or a '35mm equivalent focal length' of a 52.5mm) on a 'Full Frame' body.

Plus, if a DX lens were labeled with it's 35mm equivalent angle of view, you wouldn't be able to correlate the focal lengths of FF lenses with the focal lengths of DX lenses. Since DX bodies can use either DX or FF lenses, using the actual focal length on one type of lens, and the 35mm equivalent focal length on another type, would be very confusing.

For more info; see 'Focal Length' and 'Crop Factor'
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jul 4, 2010, 10:45 AM   #3
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Quote:
Originally Posted by dafiryde View Post
Why is it that a Dx lens being used on a dx camera is called ?mm and is really a ?mm.
example a nikon 35mm lens is really a 52.5mm lens, jump high, jump low it is a dx lens no matter what kind of nikon dx camera it is put on, it will always be a 52.5mm lens, so why not just call it that ?
It's not a 52.mm lens. It's focal length is still 35mm. ;-)

If you use a smaller sensor or film size, the angle of view will be narrower (more apparent magnification) for any given focal length.

If you use a larger sensor or film size, the angle of view will be wider (less apparent magnification) for any given focal length.

The only reason to even have a so called crop factor or focal length multiplier is so that users familiar with using lenses on 35mm cameras have a better understanding of how angle of view compares using a DSLR with a sensor smaller than 35mm film.

If 35mm cameras were not so popular, there would be no need to use them at all.

Since you have the same lenses for use on 35mm or smaller sensors with DSLR models from Nikon, Canon, Olympus and Sony giving angle of view is more difficult (since you don't know the camera the lens will be used on with most designs).

Nikon started giving Angle of View for DX lenses assuming an APS-C size sensor would be used (since their DX series are similar to Canon's EF-S series lenses, or KM/Sony's DT lenses, or Tamron's Di II lenses, or Sigma's DT lenses) and they will only work on a camera with a sensor smaller than 35mm film without vignetting).

For example, the Nikon specs for the widest angle of view for the Nikkor 18-70mm f/3.5-4.5G ID-IF AF-S DX lens is shown as 76 degrees at a focal length of 18mm (as you would expect when a lens with a focal length of 18mm is used on a DSLR model with a smaller sensor).

If you look at a non-DX lens, the angle of view shown in the specifications for a given focal length assumes it will be used on a 35mm model.For example, the Nikkor 18-35mm f/3.5-4.5D ED-IF AF lens (non DX lens) shows an angle of view in it's specifications of 100 degrees at a focal length of 18mm (as you would expect when an 18mm lens is used on a 35mm camera).

The focal length at the widest setting for both lenses is identical. What changes is the angle of view, depending on the size of the sensor/film the lens is being used with.

If you used the non-DX lens (designed for 35mm cameras) on a DSLR, the angle of view would be identical to the DX lens (digital only lens design) for the same focal length setting.

Since we have lenses that can be used on cameras with more than one sensor or film size, it's tougher to give angle of view for these (although they could give multiple angle of views in the specs, showing it for multiple sensor/film sizes).

If 645 format was more popular than 35mm, we may have be seeing "focal length multipliers" to help medium format users make the transition to 35mm, so that users could understand that a lens will have more apparent magnification (narrower angle of view) for any given focal length when used on a 35mm camera versus a medium format model. ;-)


If you really want to know the formula, here it is:
  • Angle of View = 2 * ArcTan(Film Dimension / (2 * Focal Length * (1 + Magnification)))
If you want some simple formulas for how larger format films compare to 35mm for a given focal length lens, here some are::

645 focal length x 0.62 = equivalent 35mm focal length
6x6 focal length x 0.55 = equivalent 35mm focal length
6x7 focal length x 0.48 = equivalent 35mm focal length

If you use a lens designed for a given format (as in the new "digital only" lenses designed for a DSLR with a smaller sensor), you don't crop it (as the image circle is designed to match up to the film or sensor). But, you still have to use the exact same formulas for angle of view comparisons for a given focal length lens.

For example, if you use a 50mm lens on a 645 format camera, you'll have a wider angle of view (less apparent magnification) compared to a 50mm lens on a 35mm camera (it would be like using a 31mm lens on a 35mm camera from an angle of view perpective). You'll have a wider angle of view on 645 film.

Or, to put it another way, you'd have a narrower angle of view (more apparent magnification) using any given focal length lens on a 35mm camera versus a medium format model. You'd need to multiply the focal length of a lens used on a 35mm camera by around 1.7x to determine the focal length you'd need to get the same angle of view on a camera using larger 645 film.

A lens designed for a 35mm camera will behave exactly the same on DSLR from an angle of view perspective as a digital only lens of the same focal length on that same DSLR from an angle of view perspective (apparent magnfication, what you see on the resulting image).

You'd still need to multiply the actual focal length of the lens by 1.5x if a lens is used on a DSLR with an APS-C size sensor to see what focal length you'd need on a 35mm camera for the same angle of view.

Where the confusion comes in, is because lenses designed for a 35mm camera have a larger image circle compared to lenses designed for a DSLR with an APS-C size sensor.

That doesn't have anything to do with the angle of view you see for a given focal length lens. In the case of a lens originally designed for a 35mm camera, the extra space in the image circle just isn't used with a DSLR using an APS-C sensor.

Again, the actual focal length of the lens is still the same in your example (35mm). You'll just have a narrower angle of view because of the smaller sensor size.
JimC is offline   Reply With Quote
Old Jul 4, 2010, 12:06 PM   #4
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,572
Default

The 35mm equivalent focal length is just so someone can compare the results obtained from one camera with the results from a different camera. If you're only interested in what happens with different lenses on the same camera, it doesn't mean anything.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jul 5, 2010, 4:59 AM   #5
Senior Member
 
Join Date: Apr 2004
Posts: 688
Default

Quest #2
understanding lens Quality
i keep reading that a lens is sharp in the center but soft around the edges no comprend

Dave
Trini
dafiryde is offline   Reply With Quote
Old Jul 5, 2010, 6:02 AM   #6
Senior Member
 
Join Date: Apr 2004
Posts: 688
Default Quest #3

in Vr11 nikon claims 3-4 stops, is this full stops, 1/3 stops, 1/2 stops

Dave
Trini
dafiryde is offline   Reply With Quote
Old Jul 5, 2010, 7:01 AM   #7
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,572
Default

Quote:
Originally Posted by dafiryde View Post
Quest #2
understanding lens Quality
i keep reading that a lens is sharp in the center but soft around the edges no comprend

Dave
Trini
The more a lens has to bend light, the harder it is to focus correctly. Light coming straight in doesn't have to bend very much, but light coming from the edge of an image has to be bent a lot more.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jul 5, 2010, 7:08 AM   #8
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,572
Default

Quote:
Originally Posted by dafiryde View Post
in Vr11 nikon claims 3-4 stops, is this full stops, 1/3 stops, 1/2 stops

Dave
Trini
Image stabilization prevents (or at least reduces) motion blur due to camera shake. That means you can use a slower shutter speed than you would normally. Nikon bills their VR-II system as providing an additional 3-4 stops advantage over not having a stabilization system. That's whole stops, but they can't be cashed in for aperture, only shutter speed. That is, with a particular lens in a particular situation, if you need to use a shutter speed of 1/250 without stabilization, with VR-II, you can go down to 1/30 or even 1/15 without getting motion blur due to camera shake.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Jul 5, 2010, 7:25 AM   #9
Senior Member
 
Join Date: Sep 2005
Posts: 1,093
Default

Quote:
Originally Posted by dafiryde View Post
Quest #2
understanding lens Quality
i keep reading that a lens is sharp in the center but soft around the edges no comprend
This means exactly what it says -- that you can see more detail in the center of the lens than you can at the edges. Some lenses are better at showing detail than others. And some lenses are good at showing detail in the sweet-spot of the lens but get less distinct as you move to the edges.

This aspect of a lens is quantified in terms of "MTF," or modulation transfer function. You will see listing of the "line pairs of resolution" or something similar. On sites like slrgear.com, you will see photographs of targets that have line groupings in the center and corners that allow you to determine this value. Basically, you just look and see what are the smallest pairings of lines that you can read at each of the regions of the lens (there is a more quantitative way to do it, but that's ultimately the idea.) You meaure this value for each lens at a variety of zooms and apertures -- it will vary with both things.

The quality of a lens is very tricky to characterize, even in terms so quantitative as "sharpness." It is not uncommon for one lens to be sharper in the middle than another, but softer at the corners for a given focal distance and aperture. In that case, which is the "sharper" lens? Add in different responses at different apertures or zooms, and you have a very hard property to evaluate meaningfully in a review. The industry has evolved complicated graphs that will help you, but it's a complicated business to put all this together into a single "recommended" or "highly recommended," etc. Add in distortion, vignetting, CA, and the less quantifiables like "bokeh" and "contrast" and you can appreciate how hard it is to give an accurate picture of a lens in a review.
tclune is offline   Reply With Quote
Old Jul 5, 2010, 11:12 PM   #10
Senior Member
 
Join Date: Jun 2004
Posts: 477
Default

Quote:
Originally Posted by dafiryde View Post
Quest #2
understanding lens Quality
i keep reading that a lens is sharp in the center but soft around the edges no comprend

Dave
Trini
Or to put that in the simplest possible terms:

Sharp = clearly visible with good detail
Soft = Not clearly visible, and with not much detail

Suppose you look at the back of a Coke can.... you can read everything on it. Now take it underwater, and try to do the same. Even though your eyes have focused on the can, the additional bending of the light underwater makes the back of the can not clearly visible, and with not much detail.
dnas is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 2:06 AM.