Full Frame Worth the Upgrade, Lens equivalence is not just about focal length

July 30, 2015  •  Leave a Comment

I've not fully finished this article but in reviewing it I kinda felt like it was probably too technical...  I've opted to not finish it but wanted to publish it for any and all who were still interested.  I hope you enjoy it.  

So if you're wondering if full frame really is the holy grail that its cracked up to be.  Or, if you should just stick with your current crop sensor camera.  This is another peek into some of what to expect on the other side.  Many of you know about crop factor and effective focal length but don't understand how aperture plays into this.  So don't miss out on this article thinking that I'm only talking about focal lengths!  Also, if you just upgraded to full frame and you're wondering why the field of view and depth of field of your lenses aren't at all like you're used to this article is for you!  This is also something you want to use when comparison shopping online.  Is that Panasonic 42.5mm f/1.2 lens really worth $1400 dollars?  Maybe not once you realize you can get just about the same thing out of a $217 dollar one.  Or you can get something even better out of a $500 dollar one...  Knowledge and understanding can save you a LOT of money!

I don't expect (or want) to convince you that full frame is better.  I also don't want to convince you to stick with your crop sensor camera.  As you will come to know from me i hope to inform you so that you may make the decision yourself.

Ok so lets say you have your APS-C sized crop sensor DSLR camera.  What you need to know is your "crop factor".  The two commonly known values are that Canon's have a 1.6x crop factor and Nikon's have a 1.5x crop factor.  This means that Nikon's APS-C crop sensor cameras are just a tiny bit larger than Canon's APS-C crop sensors.  

Effective Focal Length

So many of you know that this crop factor applies to the equivalent focal lengths when used on these cameras.  For now I'll just use Nikon's crop factor of 1.5x as its easier to do the math.  For example if you were to mount a 35mm lens onto your crop sensor camera you will get the same field of view through that lens as a 50mm lens on a full frame camera.  The math is simple.  You take the 35mm lens and multiply that by the crop factor.  

35mm * 1.5 = 52mm.  

Thus a 35mm on a Nikon D5300 for example will have the same field of view as a 50mm lens on a Nikon D750.  All lenses are specified as if they are mounted to a full frame camera.  So lets do some more equivalences of some common lenses.  This table essentially tells you what these lenses should be called if they are mounted to a crop sensor camera:

       35mm = 52mm  (A slightly wide angle lens that becomes a "normal" lens on a crop sensor)
       50mm = 75mm  (a "normal" lens that becomes a slightly wider classic portrait lens)
       85mm = 128mm  (the classic portrait lens)
  18-55mm = 27-83mm  (kit lens included with some of the entry level Nikon's)
18-105mm = 27-158mm  (kit lens included with the D7000)
18-140mm = 27-210mm  (kit lens included with the D7100)
  24-70mm =  36-105mm   (my favorite lens to use on crop sensor cameras)
70-200mm = 105mm-300mm   (my favorite lens to use on full frame senor cameras)
70-300mm = 105mm-450mm   (the 70-300 f/4.5-5.6 is a great lens to add to a kit lens to get more telephoto capabilities)

Hopefully this helps you understand when you mount a lens onto your crop sensor camera what you should think of as far as field of view.  But this is *NOT* the whole picture.  You also need to think of the effective aperture to make an informed comparison!!!

Effective Aperture

Many photographers understand effective focal length but don't understand that there is also an effective aperture component to using a crop sensor camera.

This is something that not many photographers know or understand.  But its something that is hinted at often.  How its hinted at is when you are in the camera store the salesman will tell you that full frame will get you more bokeh or that it will blur backgrounds better.  Is this true?  Yes and no.

If I was to mount a lens onto a tripod.  Assume its a collared lens that I can mount to a tripod directly.  Ok now assume that I focus this lens on a subject and shoot with the lens wide open.  Now lets assume I put a crop sensor camera onto this lens and take a picture, the replace this camera with a full frame camera body and take another picture.  Take a look below at the example:

<fixed lens position example>

As you see in the above image the full frame captured a wider field of view but the depth of field on the two images is identical.  No additional bokeh or reduction in depth of field has occurred.  The reason why is because nothing changed in the lens.  The focus stayed the same the zoom setting stayed the same and its position was fixed because it was on a tripod.  Essentially the same light comes through the lens and the camera captures it.  The camera doesn't change how the light comes through that lens and thus the depth of field remains identical.  So was the guy at the store just lying to us?  Not exactly.

What happens is that when we take pictures of subjects we often frame them similarly.  So now lets ask ourselves what makes for a narrow depth of field?  First thing everyone thinks of is a large aperture (aka a small f number).  But its also effected by getting a longer focal length lens and/or getting closer to your subject.  OK so to say that again the 3 factors that play into depth of field are: Aperture value, Focal length of our lens, and Distance to subject.

With the knowledge of what effects depth of field and the fact that we usually frame things similarly.  When you go between a crop sensor camera and a full frame camera; you will find that you either zoom in more or take the shot from a closer distance to frame the image similarly.  In other words if you use an 85mm lens on a crop sensor camera you will need to zoom into 128mm on a full frame camera or you will have to stand much closer with the same 85mm lens.  This similar framing is going to reduce your depth of field.  

Now for the cool part.  Rather than thinking of how the distance or the focal length effects your depth of field you can use your crop factor and your aperture.  In other words if you were to take a shot with your crop sensor camera at 85mm f/1.8 that would have about the same depth of field as if you were to shoot it on a full frame at f/2.8.  If you're wondering how I got that its the f/1.8 times the crop factor of 1.5.  1.8 * 1.5 = 2.7.  Then I rounded to the nearest stop value of f/2.8.  

Now for a bit of clarification.  This aperture equivalence only accounts for depth of field.  When it comes to light gathering capability and the exposure triangle they are all aperture values are equivalent regardless of crop factor.  So the same aperture ISO and shutter speed will generate the same exposure brightness on both camera's but there will be less depth of field on the full frame camera (if you frame the two images similarly).  Hopefully that makes sense.

Examples:

Lets say you buy a 50mm f/1.8 lens to use with your Nikon D5300.  This lens would be the equivalent of 75mm f/2.8 if you were to compare to a full frame camera.


Now lets say you're using a micro 4/3 camera with a 2X crop factor and you plunk down about $1400 for a brand new Panasonic 42.5mm f/1.2 lens.  Thats amazingly fast you think the depth of field will be razor thin on that.  However, when you see this lens you should think of it as the same as a 85mm f/2.4.  For years everyone has considered 85mm to be the classic portrait lens.  That is why Panasonic so specifically made a 42.5mm lens.

Another way to get this similar image quality would be to use Nikon's 50mm f/1.8 on a DX crop sensor body.  The 50mm x 1.5 becomes 75mm (not quite 85mm but close) and the 1.8 x 1.5 aperture becomes 2.7 (not quite the same but very close).  But also remember you're comparing a $1400 dollar lens to a $217 dollar one.  An 85mm f/1.8 lens to use on a full frame camera costs about $500 but it is actually able to be used at a full frame f/1.8 and thus is capable of much narrower depths of field than the other lenses being compared here.

I can here skeptical people asking why would someone even make a 42.5mm f/1.2 lens for $1400 dollars if it was about the same as a $217 dollar lens.  The answer is because people will buy it!  The micro 4/3 camera's are a lot lighter and people enjoy their convenience.  The people who buy these cameras want to have that 85mm equivalent focal length.  f/1.2 was the fastest Panasonic could make the thing without the cost going totally through the roof.  Nikon's 50mm f/1.8 lens is MUCH cheaper because its WAY easier to manufacturer.  But its bigger and heavier to carry around.  


Now lets talk about a zoom lens.  My favorite go to lens is my Nikon 70-200 f/2.8 VR.  If I've got the room to back up and get what I want this is probably going to be the lens on my camera.  This is not a cheap lens by any stretch.  The new VR2 version costs $2400 dollars.  If you wanted to get this lens for micro 4/3 you'd need to get a 35-100mm zoom.  A quick search revealed that Olympus makes a 35-100mm f/2 for micro 4/3.  This lens costs $2480.  But does it compare to my trusty 70-200 f/2.8 really?  That f/2 when put on a 2X crop factor is actually an f/4 lens.  As it turns out Nikon actually also makes a 70-200 f/4 that is a much cheaper alternative for only about $1400.  Its all about knowing how to compare things on a level playing field.

If you have any questions feel free to contact me or post a comment!

 


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