Go Back   Steve's Digicams Forums > Digicam Help > Add-On Lenses

Reply
 
Thread Tools Search this Thread
Old Sep 5, 2003, 2:36 PM   #1
Member
 
Join Date: Sep 2003
Posts: 71
Default Digital Camera Lens Question

Newbie here - with probably a dumb question but........

If I am a 200MM lens - why does that lens act like a 300MM lens on a digital camera?

What does this magnication increase do to the F stop? Make the lens slower??


If I had a 100MM-300MM lens - would that get bumped up by 50% as well??

Thanks
Eric
eriseman is offline   Reply With Quote
Sponsored Links
Old Sep 5, 2003, 8:44 PM   #2
Senior Member
 
Join Date: Jul 2003
Posts: 156
Default

I'm not super knowledable about it but perhaps I can point you in the right direction untill someone else gives a more qualified answer. I assume that you are talking about dslr's here, otherwise I have no idea what you are talking about.
What causes the lens to act at a higher magnification is the fact that sensor is smaller compared to the size of film. You basically have a factor that you multiply the lens size by to get the equivalent for your camera. That would mean that a 100mm-300mm lens would get bumped up by 50% if 50% is what corelates to the particular camera. Diffrent dslr cameras use diffrent size sensors so the amount of change will vary from camera to camera.
The smaller the sensor size, the larger the depth of field will be from the same f-stop. You can see this in prosumer cameras that have a substantially larger depth of field from the same f-stop because the sensors are much smaller. As far as exposure I think it is the same but I am not certain on that point.
I hope that makes is a little clearer untill someone more knoledgable responds.
richardh is offline   Reply With Quote
Old Sep 5, 2003, 9:06 PM   #3
Senior Member
 
Join Date: Oct 2002
Posts: 579
Default Re: Digital Camera Lens Question

Quote:
Originally Posted by eriseman
If I am a 200MM lens - why does that lens act like a 300MM lens on a digital camera?
I would assume that the camera you were using is a digital camera with interchangeable lenses. If this is the case, due to the smaller sensor size, the sensor only catch about 50% of the area of a 35mm film frame. This means the angle of view recorded by the sensor is smaller and hence giving you a longer focal length. Each of these non-full-frame cameras has a multiplier to be used for computing the effective focal length of a lens. For examples, Nikon's cameras have multiplier 1.5, which means a 200mm lens becomes 200*1.5=300mm on a Nikon digital camera.

Quote:
Originally Posted by eriseman
What does this magnication increase do to the F stop? Make the lens slower??
It has no impact on F stop and the lens has the same speed. This multiplier only changes the focal length to accommodate the smaller sensor size.

Quote:
Originally Posted by eriseman
If I had a 100MM-300MM lens - would that get bumped up by 50% as well??
Yes, if the multiplier of your camera is 1.5, that 100-300mm zoom becomes a 150mm-450mm zoom.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 User Guide
shene is offline   Reply With Quote
Old Sep 6, 2003, 6:59 PM   #4
Moderator
 
Join Date: Jun 2002
Posts: 1,139
Default

It's a little more complicated than most of the responses indicate, but in a nutshell the optical properties remain identical regardless of the so called "crop factor," but there is electronic magnification because of the fact that you are vesting the full resolution of the sensor within a smaller (more narrow) field of view than a full sized 35mm sensor.

Because the numeric values derived from information gathered by the photo diodes is used to create pixels, and because these pixels all have a common size in print or on the display magnification equivalent to the 1.x factor of the crop (or more accurately "reduced field of view") occurs to the degree of the difference between a full frame field of view and the reduced field of view as "seen" by the sensor. This can only happen within the resolution capacity of the lens itself. Think of it like peering into the actual resolution capabilities of the lens with an electronic magnifying glass.

Here's a link to my web site where I have a visual and more detailed explanation of this:

http://www.lin-evans.com/cropfactor/cropfactor.htm

Lin
Lin Evans 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 6:42 AM.