Why the crop factor also affects depth of field

Monday 20 June 2022, 20:40 by Nando Harmsen | 6,029 views | 7 comments

This article was translated from Dutch with the help of AI. Read the original.

A crop sensor is simply a sensor with a smaller surface area than a full-frame sensor. You should not look for more behind it than that.

The consequence is that you do not use the full angle of view of your lens. That indirectly happens to be the reason why depth of field is different with a crop sensor. I explain it in this article.

There is quite a lot of confusion about the influence of a crop sensor on depth of field. Some say it has an influence, others do not. Open the first DoF calculator you find and you will see the depth of field change when you switch from a full-frame sensor to a crop sensor.

A crop sensor is simply a smaller sensor that captures a smaller part of the image. If a full-frame sensor captures the large photo, a 1.6x crop sensor will capture only the part outlined in red.
The result from a DoF calculator seems like proof enough, does it not? Yet it is not that simple. Yes, the depth of field changes, but for different reasons than you might think. The reason is that the wrong starting point is often used.

Switching between full-frame and crop

What happens when you switch between a full-frame sensor and a crop sensor? The angle of view changes. In other words, the image captured by a crop sensor corresponds to a longer focal length on a full-frame sensor.

The image from 85mm on a full-frame camera, with f/5.6 and at a distance of 4 metres.
To show this in numbers. An 85mm lens on a crop sensor that is 1.6x smaller produces the same image as a 136mm lens on full-frame (85 x 1.6). We all know that, right?

Of course, we have to view both photos at the same dimensions, for example full screen on a Full HD display or as a photo print.

The image from 85mm on a 1.6x crop camera, with f/5.6 and at a distance of 4 metres. Because the sensor is smaller, the angle of view is smaller and the subject is shown 1.6x larger. Compare this with the previous photo.

Keep the subject the same size in the frame

Let us look at it from a different perspective. The subject photographed with 85mm on a crop camera is shown 1.6x larger than with 85mm on a full-frame camera. So you get a very different shot.

If you want the subject to be the same size in the frame with the crop camera as with the full-frame camera, you can do two things. You can make the distance 1.6x greater, or choose a shorter focal length. In this example, that is 50mm (85 / 1.6, rounded to 50mm).

By reducing the focal length by (approximately) 1.6x to 50mm, the subject is the same size in the frame with a 1.6x crop camera as with 85mm on full-frame. The distance remains 4 metres, the aperture is f/5.6
Now something remarkable happens. In the first case, the distance becomes greater, which makes the depth of field smaller. In the second case, the focal length becomes shorter, which also makes the depth of field smaller.

A crop factor only indirectly affects depth of field

In short, the smaller depth of field that results from photographing your subject with a camera with a crop sensor is caused by either having to stand further away with the same focal length, or using a shorter focal length.

So the depth of field is caused indirectly by the crop sensor.

The difference between a full-frame sensor and 85mm on the left, and a 1.6x crop sensor and 50mm on the right. Both were photographed from a distance of 4 metres and at f/5.6. The difference is not caused by the crop sensor, but by the difference in focal length.

What if you crop the full-frame image?

To complete the story, as it were, we can compare the depth of field between a crop sensor and a full-frame sensor.

We keep the focal length, distance and aperture the same. However, we do crop the full-frame photo until the subject is the same size as with the crop sensor. You will see that there is no difference in depth of field.

When I digitally crop the photo taken at 85mm on full-frame to the same magnification as with 85mm on the 1.6x crop camera, you can see that the depth of field is the same at the same distance from the subject and with the same aperture. Can you tell which one was photographed with the crop sensor, and which one was digitally cropped from a full-frame image?

Conclusion

The crop sensor has no direct influence on depth of field. The influence is indirect, because we have to use a shorter focal length or a greater distance to keep the subject the same size in the frame.

If we keep the focal length, distance and aperture the same, there is no difference at all in depth of field.

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About the author

Nando Harmsen has been photographing for more than 40 years. He specialises in wedding photography and landscape photography. Nando also gives talks, workshops, masterclasses and photo trips, and regularly reviews new and interesting equipment.

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7 comments

Donald Willemsen wrote on Tuesday 21 June 2022 at 10:48 | reply
Interessant verhaal, maar er is me toch wat opgevallen wat het verhaal in mijn ogen doet wankelen.
Bij de laatste 2 foto's net boven de conclusie zitten duidelijke verschillen. Kijken we naar het onderste deel van de lamp dan zien we rechts ernaast bij de ene foto meer geel uit de achtergrond dan bij de andere foto. De digitale crop van de fullframe is in mijn ogen niet gelijk aan de cropsensor. Een andere oorzaak kan nog zijn dat de lamp niet exact op dezelfde plaats hangt ten opzichte van de achtergrond. De twee foto's zijn niet exact gelijk qua uitsnede.
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Dies Groot wrote on Tuesday 21 June 2022 at 11:00 | reply
Bij dit diafragma, deze sensorgrootte's en deze resultaten heb ik toch mijn twijfel of dit wel exact 4 meter afstand was en toch niet iets dichterbij ...?
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Nando wrote on Tuesday 21 June 2022 at 11:44 | reply
@Donald Willemsen:
De foto's zijn allemaal uit de hand gemaakt, niet vanaf statief. Dat betekent dat er gewoon wat kleine verschillen zijn van compositie
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Nando wrote on Tuesday 21 June 2022 at 11:46 | reply
@Dies Groot:
Exact 4 meter is het waarschijnlijk niet. Wel ongeveer. Ik heb er geen meetlint bij gehouden.
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Matt wrote on Tuesday 21 June 2022 at 16:59 | reply
Een andere benadering is naast equivalente brandpuntsafstand, ook een equivalent diafragma.

Dus als je op full frame met 85mm, F/5.6 fotografeert, gebruik je op APS-C 50mm, F/3.5. Dan krijg je ook een identieke foto qua scherpdiepte.
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Peter Van Himste wrote on Tuesday 21 June 2022 at 17:59 | reply
Je zou eens een gelijkaardig verhaal kunnen brengen over diafragma: waarom een f/2.8 lens op een crop camera niet zomaar een full frame f/4 lens is. Daar is ook heel vaak verwarring over.
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Jos wrote on Tuesday 21 June 2022 at 21:55 | reply
Ik heb indertijd een vergelijk gemaakt met een Canon 5D en een Olympus E-5. Voor beide toestellen heb ik dezelfde lens gebruikt: een 50mm f/1.4 Zuiko (OM-systeem).
Om het onderwerp even groot op beide sensoren te krijgen, moest ik dus met de Olympus op een grotere afstand gaan staan, waardoor de scherptediepte groter werd. Dit uiteraard met ook hetzelfde diafragma (f/5.6).
Even dit: naarmate we dichterbij ons onderwerp komen, gaat de scherptediepte kleiner worden. Dat geldt voor ieder objectief. Ook is de scherptediepte niet afhankelijk van de brandpuntsafstand, maar van de effectieve lensdiameter.
Zo zal een 50 mm f/4 (= diameter 12.5mm) een even grote scherptediepte leveren als een 200 mm bij f/16. Uiteraard hoort dan ook de afstand even groot te zijn, want zoals gezegd beïnvloedt die ook de DOF.
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