Single-Lens Reflex

SLR. Surely, you are familiar with those letters. But do you know what they stand for? Click through to find out and learn about the physics behind it. Optics is the reason you can look through your SLR viewfinder and see exactly what your photograph will look like.



DSLR. Digital SLR. Digital Single-Lens Reflex. What does this mean? In this post I explain the optics of an SLR and compare it to viewfinders from mirrorless cameras.

A single-lens reflex camera uses a mirror (hence the term reflex) and a pentaprism that allows the photographer to look through the lens and see exactly what will be captured in the photograph. A digital single lens reflex camera is a camera that combines the optics of an SLR with a digital imaging sensor.

Photos by Canon
Photos by Canon


I own a Canon 550D, which is a DSLR. However, I also own a Fuji X100S, which is not a DSLR. The FujiX100S does not have a mirror (hence why it’s called a mirrorless camera) and when you look through the viewfinder you do not see exactly what will be captured in the photograph.

In this type of mirrorless cameras the viewfinder is located close to the lens but it is a little bit displaced, hence what you see when you look through the viewfinder is slightly different than what will be captured in the photograph. This difference is more apparent the closer the object is to the camera. Another disadvantage of this type of viewfinder is the fact that if you change the focal length of your lens, it will not affect what you see through the viewfinder.

Note: for the purpose of this explanation I will always be referring to the optical viewfinder of mirrorless cameras, I do know that the FujiX100S has an electronic viewfinder but I will not be talking about that in this post.

Photos by Fujifilm
Photos by Fujifilm


Now that we have extablished what an SLR is, let’s look at now it functions. In the diagram below we can see that a light ray originating from our subject will enter the camera through the lens and once inside the camera body it will encounter the reflex mirror. This mirror has two positions: in it’s upper position it lets the light through towards the shutter and the film/sensor. However, when the mirror is in place (as pictured in the diagram) it deflects the light upwards, past the condenser lens, through the pentaprism and out the viewfinder into the photographer’s eye.


So, essentially, we are taking the light that comes in through the lens and making it go towards the viewfinder so we can see exactly what will come up in the photograph. The condenser lens is there to adjust the image size so it fits in the viewfinder. But why do we need a pentaprism? It seems like an awful lot of trouble when we could just use a second mirror instead and make the light go to the viewfinder too!


While a simple second mirror might seem like a better idea than a complex pentaprism, it is not in fact so. Since the front mount lens inverts the image of our object, if we use two mirrors to direct the light towards the viewfinder we will see everything upside down!! A pentaprism adds two extra reflections in the light’s path, thus ensuring that the image comes upright out of the viewfinder and into the photographer’s eye. This pentaprism is sometines called a penta-mirror, because the two sides that reflect the light are coated to provide mirror surfaces (since there is no total internal reflection at that angle).


Based on the fabulous optics of SLRs and the fact that they allow the photographer to view exactly the shot they are just about to take one may wonder why anyone would choose a mirrorless over an SLR. In fact, back in the film days, before digital camers, I’d say that SLRs were the most “accurate” cameras (both waist-level finders and pentaprism models).

Me using my Canon 550D DSLR. Photograph by Francis Gorrez (
Me using my Canon 550D DSLR. Photograph by Francis Gorrez (

However, nowadays with electronic technologies and digial imaging, there are other ways of viewing exactly the shot you are just about to take. You can just display the camera sensor on the screen on the back of the camera and voila! What you see is what you get :) This, coupled with great advances in focusing technology, and the reduced size of mirrorless cameras (they have less components to fit inside!) is the reason why mirrorless cameras are so popular today!

Lucia using my mirrorless FujiX100S.
Lucia using my mirrorless FujiX100S.

Thank you for reading! Feel free to leave a comment here or on my Facebook page, I read them all and really appreciate them.

xx Ana :)

Ps. Check out my new post “DSLR Shutter in Slow Motion” to see a slow motion video of the SLR mirror mechanism in action, it’s very cool!!


6 thoughts on “Single-Lens Reflex

  1. […] My last post was about the physics of a Single-Lens Reflex. Wether film or digital, the mechanim that allows you to see through the lens is the same. In summary, there is a mirror that deflects the light to the viewfinder while composing for the shot, and when you press the shutter this mirror moves up to let the light go through the shuter and onto the film/sensor. […]


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