Nick Miners Photography

Monday, April 12th, 2010

One of the ques­tions you may ask your­self when decid­ing to buy a digital SLR for the first time is ‘Full Frame or cropped sensor?’ Even if it isn’t, it should be. The obvi­ous next ques­tion is: what’s the dif­fer­ence? Well, it’s more com­plic­ated than you might think.

A full frame (‘FF’ here­after) cam­era has a sensor that is the same size as a frame of 35mm film (36mm x 24mm); so-​​called ‘cropped sensor’ (‘CS’) cam­eras have smal­ler sensors, though there is no single stand­ard, for example the sensor in a Canon EOS 550D meas­ures 22.3mm x 14.9mm whereas in a Canon EOS 1D mk IV it’s 27.9mm x 18.6mm. Gen­er­ally, FF cam­eras cost con­sid­er­ably more than CS mod­els, how­ever you might be able to pick up a second hand Canon EOS 5D or Nikon D700 for a reas­on­able price.

Many people talk about the ‘mag­ni­fy­ing effect’ of a CS body. I find this term mis­lead­ing; any mag­ni­fic­a­tion is done by the lens, and a 50mm lens is a 50mm lens on any cam­era. The image pro­jec­ted onto the sensor plane is identical, but the smal­ler sensor means only a part of the image is cap­tured com­pared to a FF cam­era. To phys­ic­ally print the image would require more mag­ni­fic­a­tion but this is applied after the pho­to­graph is taken, and so is not tech­nic­ally a func­tion of the sensor.

This also means that to get the equi­val­ent field of view in a CS cam­era to a 50mm lens on a FF model, you would need to user a wider lens, eg 35mm, though the exact focal length would depend on the phys­ical dimen­sions of the sensor. Using a lens with a shorter focal length means you get more depth of field, so this is one of the first things you need to think about when choos­ing sensor size; does your style of pho­to­graphy require nar­rower or wider DoF? Below 35mm though, it must be said, DoF is gen­er­ally so wide that it makes little difference.

Crop­ping the image also means it is easier to fill the frame with a sub­ject, espe­cially use­ful in wild­life pho­to­graphy where small anim­als are shot using tele­photo lenses of 300mm or more. This is where it is bene­fi­cial to use a smal­ler sensor; while it is also pos­sible to crop a FF photo in post pro­cessing to achieve the same effect, you end up wast­ing pixels and los­ing detail as the res­ult­ing res­ol­u­tion is so much lower.

Of course the con­verse applies to wide angle lenses. Much of the drama of a good land­scape comes from using lenses of 20mm or less, and when the extremes of a pho­to­graph are cropped out, this added drama can be lost. Unlike with tele­photo lenses how­ever, you can­not reverse the effect in PP.

The one final con­sid­er­a­tion is image qual­ity. Gen­er­ally, except for the most expens­ive lenses, this deteri­or­ates towards the edge of the pic­ture, so by using a cropped frame sensor you are elim­in­at­ing the less well defined areas of your image mean­ing you don’t neces­sar­ily need to spend as much on good qual­ity glass. On the other hand (though this is less true with the latest devel­op­ments in sensor tech­no­logy), the high ISO per­form­ance of CS cam­eras is not as strong as that in FF cam­eras. This is because the pixels are more densely packed in a CS cam­era, and so are smal­ler and more sens­it­ive to non-​​optical factors, such as elec­tronic inter­fer­ence caused by the camera’s own circuits.

It is not my inten­tion to say what is right or wrong, as I don’t believe there is such a dis­tinc­tion. But if you are fully aware of the con­sequences of your choice of sensor size, hope­fully it makes it easier for you to make the decision that works for you.

Tags: crop, full-frame, lens, sensor