Effective focal length

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Effective focal length

Postposted on Fri Jan 24, 2014 8:05 pm

I read an article somewhere in which Nikkor 18-200mm f/3.5-5.6G DX is used on Nikon D5100 so that the effective focal range is 27-300mm. MY question is under what circumstance the range is as stated in the specfications or 18-200mm?
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 8:17 pm

The 5100 uses a DX sensor. This means that all lenses you mount onto it you have to multiply it by 1.5. So the lense you listed, 18-200mm, will have an effective minimum focal length of 27. Conversely the maximum focal length will be 300mm.

Hope that helps!
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 9:13 pm

You indicate that the effective focal range is different because the sensor of the camera is a DX sensor. Then what if a full frame camera is used? Isn't the x1.5 multiple factor still applicable? My question is how can that lens be used as a 18-200mmm lens?
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 9:16 pm

The stated range for lenses assumes a full-frame sensor. So on an FX camera -- or on any old film SLR -- an 18-200 lens is an 18-200 lens. On a camera with something other than a full-frame sensor, you have to multiply by the crop factor (1.5 for Nikons, 1.6 for Canon I believe; it may be different for others).
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 10:36 pm

Do you mean the full area of FX camera sensor is used even when the DX lens is on the camera? Isn' it that the size of the image by DX lens is smaller than FX sensor?
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 11:11 pm

A full frame body will never be able to use a cropped sensor lens to its fullest. It will either have terrible vignetting or it will automatically crop it down to a cropped sensor size. Think in terms of your body only: you know that it is a cropped (aps-c) sensor so multiply the lens focal length by 1.5. So an 18mm lens is essentially the equivalant of 18 x 1.5 = 27mm on a full frame body (if the lens were a full frame lens).

I just recently went full frame, so I have to do worse mental adjustments. I have ti multiply by .66. So when I think of my 24mm lens, I have to remember that it is equal to a 16mm on my old cropped sensor. It gets rough when I have to think about 17mm. I just know its about what an 11.5mm would have been. But it is awfully jard to use a 300mm and seeing what I am used to being 200mm. I feel so far away!
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Re: Effective focal length

Postposted on Fri Jan 24, 2014 11:20 pm

I've been in the Dx/APS C world for a short enough time that I still convert backwards. My 60mm is a nice 90m view lens and I'm really looking forward to having my Leica Elmar 135mm 4 be a 135mm lens when I get the FF Sony.

The different sizes are fairly well explained here:
http://en.wikipedia.org/wiki/APS-C

And this, from there is useful:

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Re: Effective focal length

Postposted on Sat Jan 25, 2014 2:00 am

It's a shame that the chart that Pengun linked shows all of those different sensor sizes in the corner. You need to think of them as centered in the middle of the box.

A 50 mm lens is still a 50 mm lens. regardless of which camera it is mounted on. However, the camera with the APS-C sensor captures only the middle part of the image that a camera with a full-frame 35mm sensor would. The field of view is cropped to the same field of view as a lens with 1.5 times as much focal length. That's the "crop factor".

Smaller sensors are much easier to manufacture without defects, so they're less expensive. You also have more depth of field (as if you were one stop down with the longer lens on the full-frame sensor). The other beneficial effect is that many lenses perform significantly better at the center of the image than they do at the corners. APS-C captures only the sweet spot in the middle of the image.

Take a look at the MTF chart for the cheapest EF lens made:
http://www.usa.canon.com/cusa/consumer/ ... m_f_1_8_ii
Notice how the wide-open performance between 18.0 mm and 21.6 mm from the center gets pretty bad? Those are the corners of a full frame sensor. The corner of a Canon APS-C sensor extends to only 13.4 mm from the center, so all of that bad stuff at the corner of the full-frame image isn't captured by the camera. It's cropped out.

Another way to think of it is if you turned the APS-C size sensor into the portrait orientation, it would have essentially the same height as the full-frame sensor does in landscape orientation.
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 8:45 am

Thanks everyone for your replies.
Let me state what I understood:
Usable area size of the image by DX lens is smaller than that by FX lens. Based on the difference between the usable image sizes, when DX lens is used the effective focal length is ALWAYS 1.5 times the stated focal length regardless of the camera sensor size being APS-C or full frame.
Is the above correct?
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 10:17 am

With the Nikon system, it is possible to mount some lenses made specifically for APS-C sensors (DX) on some cameras with full frame sensors (FX). This isn't possible with Canon EF-S lenses (to avoid the larger mirror striking the back of the lens). Some of those Nikon cameras are smart enough to recognize that the small image circle from the DX lens isn't going to fully-cover the larger full-frame sensor, so they automatically digitally crop the image, using just an APS-C size portion of the sensor. Otherwise, the part of the image near the outer edges of the full frame sensor would be very dark (extreme vignetting).


Given that lenses are circular, we'd make more efficient use of the available image circle if our sensors were 1:1 squares rather than 3:2 rectangles. Hexagonal or octagonal prints would make even more efficient use of the lens. :lol:
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 10:39 am

churin wrote:My question is how can that lens be used as a 18-200mmm lens?


The quick answer is you can't. It requires using a sensor that's bigger than what the glass element of the lens allows.

I look at Nikon's DX lenses (and Canon's APS-C lenses) as
"this lens should have been xx-yyy mm if you could mount it on our full-frame cameras, except you can't."
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 10:50 am

If I take the aforementioned EF 50mm f/1.8 lens and I mount it on an EOS 35mm film SLR camera from 1987 or a current-day full-frame DSLR or an APS-H DSLR or an APS-C DSLR camera... it still has the same 50mm focal length that it's always had. The focal length is a physical characteristic of the lens that does not change. The field of view changes if you change the size of the sensor. The light going through the lens always does the same thing. It's just that the APS-C cameras catch a smaller amount of it.
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 11:45 am

Another thought as you look at a 50mm lens, on a medium format is a wide angle lens. On full frame, its a normal lens. On aps-c it is a short tele.

The long and short of it is that you should really only pay attention to what format your body is. The focal lenght is always the same, but it means something different on different formats.

Something that may help.was.a.thread I posted up on Dyxum about why mm and what is it measuring. It goes back to distances between 2 elements back before there would be numerous layers of glass in a lens.

http://www.dyxum.com/dforum/so-why-mms-please-educate-me_topic101290.html
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 11:59 am

Let me repeat the question in my previous post:
Usable area size of the image by DX lens is smaller than that by FX lens. Based on the difference between the usable image sizes, when DX lens is used the effective focal length is ALWAYS 1.5 times the stated focal length regardless of the camera sensor size being APS-C or full frame.
Is the above correct?
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 2:17 pm

This problem has always bothered me. A DX lens is only usable on a DX sized area of a sensor. As people have noted, on an FX sensor either the camera crops automatically, or you will have to crop manually in post processing. Either way, you get a DX sized set of pixels. And yet a DX lens is marked with full frame focal lengths. A set of focal lengths that, under no circumstances, cannot be achieved. Why would you not mark the lens with the effective focal length for the sensor it is designed to be used on. The 18-200 lense I have as the walk around lense on my D7100 will never reach 18mm.

The real problem is folks that only shoot DX and don't know any better think they have a lense with an 18mm lower end. I was reminded of this when I put an FX 24-70 on my DX body and had to sit and puzzly over why 70mm was the same field of view as on my 18-200mm. I know about the 1.5x factor and even I had to reset my brain.

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Re: Effective focal length

Postposted on Sat Jan 25, 2014 3:03 pm

churin wrote:Let me repeat the question in my previous post:
Usable area size of the image by DX lens is smaller than that by FX lens. Based on the difference between the usable image sizes, when DX lens is used the effective focal length is ALWAYS 1.5 times the stated focal length regardless of the camera sensor size being APS-C or full frame.
Is the above correct?


Yes, unless you can somehow turn off the auto-crop feature on the FX camera while the DX lens in attached.

This guy did it with interesting results.
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 3:07 pm

The focal length marked on the lens is the focal length of the glass. The focal length of the lens does not change when you swap cameras.

I believe that the real problem is that there are a huge number of people who mistakenly believe that "50mm" is a specification for field of view instead of a measurement of focal length.


If you take a look at something like a Canon PowerShot Digital Elph with a tiny 1/2.3" sensor (7.0 crop factor), you'll see that it does just fine with a zoom lens with a focal length range of 4.0-48.0mm (35mm film equivalent field of view: 28-336mm).
Last edited by JustAnEngineer on Sat Jan 25, 2014 3:13 pm, edited 1 time in total.
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 3:12 pm

JustAnEngineer wrote:The focal length marked on the lens is the focal length of the glass. The focal length of the lens does not change when you swap cameras.

I believe that the real problem is that there are a huge number of people who mistakenly believe that "50mm" is a specification for field of view instead of a measurement of focal length.


Or are confused as to just what focal length means- as with all things in photography, it's all relative :).
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 4:14 pm

Here is a couple equivilance things that may or may not help.

Micro 4/3 - 12mm
APS-C - 16mm
Full Frame - 24mm
Medium Format - 48mm

All of these are the exact same field of view. So, if you are using your aps-c camera at 16mm, for the area the picture will take, it is the same area as a 24mm on a full frame, or a 12mm on m43, or a 48mm on a huge medium format.
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Re: Effective focal length

Postposted on Sat Jan 25, 2014 7:17 pm

jihadjoe wrote:
churin wrote:Let me repeat the question in my previous post:
Usable area size of the image by DX lens is smaller than that by FX lens. Based on the difference between the usable image sizes, when DX lens is used the effective focal length is ALWAYS 1.5 times the stated focal length regardless of the camera sensor size being APS-C or full frame.
Is the above correct?


Yes, unless you can somehow turn off the auto-crop feature on the FX camera while the DX lens in attached.


Thank you for confirming my understanding.
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Re: Effective focal length

Postposted on Tue Jan 28, 2014 1:39 pm

churin wrote:Thank you for confirming my understanding.

It's a confusing topic until you've spent some time becoming familiar with the principles and nomenclature. The trick is to always remember that the "reference" field-of-view is derived from a conventional 35mm film frame. If your camera's sensor is smaller than that reference, then the image it produces is nearly the same as if you had taken a picture at the "reference" size into Photoshop, cropped a smaller area equivalent to your camera's sensor, and then enlarged it to fill the entire picture again. In other words, magnification.

So, if you have a 50mm lens installed on your Nikon with a 1.5x crop factor, the field of view it will capture is about the same as if you had installed a 75mm lens on a "reference" frame.

At the end of the day, though, it's kind of pointless to get too wound up on the distinction unless you're shooting both frame sizes back to back and trying to mimic settings for comparative purposes. Otherwise, what matters is that you spend time shooting with your camera and lenses, and obtain a real-world feel for what focal lengths and aperture settings best capture the image relative to what you wanted.
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