Do I need to upgrade to a full frame DSLR?

Many aspiring photographers often wonder… Should I get a full frame DSLR? Do I really need it? Will my photography improve if I get the 5D Mark II or D700? These are very good questions & should never be taken lightly when deciding if you really want to go full frame or be content with a crop sensor DSLR. After all, there are varied number of perceptions on this topic that your friends & colleagues may try to influence you with. No doubt the majority of folks out there believe that full frame cameras are better than crop sensor DSLRs. Or is it?

What is in a sensor size?

Sensor sizes vary in many dimentions, depending on their intented use, audience target & portability.

Full frame cameras like the 1DsMkIII/5DMkII/D700 have sensor sizes in reference to the 35mm film equivalent. Canon cameras such as the 550D/60D/7D have a 1.6X crop factor, where Nikon models such as the D300s/D7000 have a 1.5X crop factor.

Compact point & shoot cameras & mobile phones have sensor sizes between the range of 1/4″ to 2/3″. Olympus, Fuji & Kodak came up with the standard 4/3 system which has a 2X crop factor compared to a full frame 1X factor. There are also larger sensors like the medium format cameras by Hasseblad & Pentax, however these are extremely pricey.

The crop factor & focal length

The crop factor is the sensor’s diagonal size in comparison to a full frame 35mm equivalent sensor. The reason it is called this is because essentially when using a full frame lens, such a sensor will effectively crop out the sides of the captured image.

So is this necessarily a bad thing? Well… not really. Most lenses are sharpest at their centers. The quality of a lens is usually poorer at the edges. What this actually means is that a crop sensor DSLR is actually getting rid of the lowest quality parts of the image!

However bear in mind that on the flip side, if you happen to be using a low quality piece of glass (cheap 3rd party equivalents), you may be amplifying the center region for an otherwise badly constructed lens due to its limitation in resolution. Remember that the optical performance of a wide-angle lens is usually poorer than a telephoto lens.

This crop factor multiplier is also used to calculate the focal length equivalent angle of view when a full frame lens is attached to a DSLR with a crop sensor. So a 50mm lens when used with a DSLR with a 1.6X crop factor would yield a similar view as a 1.6 x 50 = 80mm lens on a full frame DSLR.

However you need to be aware that this “understanding” can be somewhat misleading. The focal length of a lens does not change just because it is plugged into a camera with a different sized sensor… it’s just the angle of view here that we are talking about. A 50mm lens will always be a 50mm lens no matter what DSLR body you attach it to.

Is bigger better?

Generally smaller sensor cameras require smaller, lighter & often slimmer looking lenses to operate on. Depending on the situation, it may be quite important to a traveller or wildlife photographer venturing into the forest.

So if you require a 300mm angle of view, you can get away by using a 200mm on a 1.5X crop DSLR, significantly reducing the amount of weight you need to carry. Shorter lenses usually cost less… and they don’t draw as much attention from onlookers especially if you are concerned that you may get robbed off your gear!

Depth of Field difference between a full frame camera vs. crop sensor camera

On a large sensor camera, the depth of field or depth-of-field will be shallower when you are trying to fill the frame with a subject of the same size at the same distance, compared to a crop sensor camera. This is because on a large sensor camera, you need to get closer to the subject or zoom-in to frame the subject. So you will need to use a smaller aperture (higher f) to maintain the same depth-of-field.

Let’s use an example. Assuming that you are using a 1.6X crop sensor camera like the 60D & you just shot a model using a 50mm lens at f/5.6…. in order to get the same depth-of-field effect on a 5D Mark II, you will need to shoot at 80mm using f/9.0.

Now let’s say you are shooting with a 5D Mark II using a 50mm at f/1.4…. in order to get the same depth-of-field, you will need to use a 60D with a 30mm at f/0.9!!! Simply impossible with any consumer lens!!

Fashion & boudoir photographers want shallower depth-of-field for portraits because of the background blur (bokeh)… where else landscape photographers want a greater depth-of-field for their scenery shots. This is why it’s difficult for a compact camera to blur the background in portraits, while large format cameras struggle to get enough depth-of-field in landscapes.

Diffraction limited aperture

Diffraction is an optical effect which can limit the sharpness of an image, regardless of the number of mega pixels your camera has. Light travels in a straight line in a consistent medium, like for example light travelling through our atmosphere. However light can be dispersed or diffracted when squeezed through a small hole such as a camera’s aperture. This is normally not noticeable however at very small apertures, you will see the effect of diffraction start to creep in. The effect is a softer looking image.

Try this… if you squint your eyes a little to see something, it actually looks clearer (sharper). But if you squint too much, things get fuzzy a little. This is diffraction!

Photographers normally use small apertures to achieve better sharpness & depth-of-field. However at some point when the aperture is too small, diffraction sets in which will offset any gain in sharpness. When this occurs, the camera optics are said to have reached its diffraction limit. Knowing & understanding this limit helps a photographer avoid softening of the image he is trying to capture & avoid having to use too long an exposure time or too high an ISO speed.

Let’s take a look at some comparisons…

DSLR with a CF of 1.6X.
Resolution = 18 mega pixels
Diffraction Limited Aperture = 8.1

DSLR with a FF 35mm sensor.
Resolution = 18 mega pixels
Diffraction Limited Aperture = 13

So we know now that the larger the sensor, the smaller an aperture the camera can use before diffraction kicks in.

Another comparison…

DSLR with a CF of 1.6X.
Resolution = 18 mega pixels
Diffraction Limited Aperture = 8.1

DSLR with a CF of 1.6X.
Resolution = 12 mega pixels
Diffraction Limited Aperture = 10

Interesting. Now the lower the number of mega pixels your camera has increases the diffraction limited aperture… meaning you can shoot with a smaller aperture on a lower megapixel camera without the image getting too soft.

If you look at both comparisons, lower pixel density increases the diffraction limited aperture. This explains why a larger sensor and/or a lower megapixel camera has an advantage. This may be a factor you want to consider when deciding if you want more mega pixels on a new camera for your intended use. More pixels does not provide more resolution for your depth-of-field requirements.

Do keep in mind that the effects of diffraction is gradual, so apertures at the diffraction limit will not all of a sudden look bad. Also this is a theoretical limit so lens quality & build has a part to play in this factor as well.

Noise & Dynamic Range

Larger sensors usually have lower density pixels, therefore larger pixels. Dynamic range means how much detail the sensor can detect light between the brightest area & the darkest area of an image. Larger pixels have greater dynamic range in general.

As larger pixels theoretically receive a higher “number” of photons during an exposure, the light signal is much stronger resulting in a smoother image with low noise. Typically if noise is viewed at a 100% crop, a sensor with a higher number of pixel count will produce a cleaner image. More pixels means the noise will appear finer & when enlarged to print size, much smoother.

Some things to consider

In real world photography, when you wish to produce an image with the same depth-of-field, a camera with the larger sensor size do not necessarily have a resolution advantage. As what I mentioned above, the difference of sensor size in relation towards depth-of-field, diffraction-limited & ISO/noise… all variables come back to even themselves up thus cameras from either a 1.6X crop factor DSLR or a full frame DSLR will ultimately produce the same depth-of-field, sharpness & noise levels when the photographer compensates for either bodied cameras.

Think about it…

On a large sensor I need to use a smaller aperture to get the same depth-of-field as a crop sensor camera. Being a large sensor DSLR, the theoretical diffraction limited aperture is smaller so this fits my purpose. Since I have to shoot a much smaller aperture than when I am using a crop sensor body, I need to control my exposure timing. To ensure a fast exposure, I’ll have to bump up ISO to get it. Hey! I have a full frame DSLR so noise levels aren’t an issue right?

On the other hand, on a crop sensor DSLR, I don’t need to use too small an aperture to get the same depth-of-field as a full frame camera. Since smaller sensor bodies have larger diffraction limited aperture, no big deal because I don’t need to use small apertures anyways. Technically I get sharper images with the lower mega pixels to shoot with. Since I’m not using such a small aperture, it’s easier to control my shutter speeds & rarely have to bump up ISO. Since I’m using low ISO, noise? What noise?

So technically to get a similar shot with the same depth-of-field, angle, framing & quality… it’s the same on both cameras.

The one thing is evident though… larger sensors can get you a shallower depth-of-field than smaller sensor cameras. So with this, crop sensor DSLRs works better for landscape photography for instance & full frame cameras works marvelous for portraits!

So going full frame helps where you require additional flexibility in shallow depth-of-field.


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3 responses to “Do I need to upgrade to a full frame DSLR?”

  1. Amit Shindore says :

    This, my friend, is an excellent writeup.

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