How can I make a photo with blurred people but a sharp background? [duplicate]

Question

This question already has an answer here:

• How can I depict vehicle traffic during daytime in a long exposure without washing out the vehicles? 5 answers
• What are neutral density filters and how do I use them to create long exposures in daylight? 3 answers

I am trying to achieve the type of photo where the people/moving parts of the photograph are virtually invisible, but the static background is perfectly sharp.

For example, I am taking a photo inside of Saint Peter's Basilica, and I want the photo to be of the beautiful architecture and decoration, not the other tourists. To achieve the effect where the people are like ghosts, I set the shutter speed to 4 seconds and let the camera figure the rest out itself.

Because of the limited lighting in the basilica, the 4 seconds are permissible and the photo comes out as desired. Here it is:

(Sorry about the image quality. It is a photo of a photo.)

How I can achieve this effect in an area with more light, like outdoors? Lowering the shutter speed alone will not help, for it will either give me a photo without the effect, or a white photo. I thought that maybe I could put a polarized filter over the lens to limit the amount of light that can enter. I also thought that maybe I can put a light gray filter on the lens to limit the amount of light but that might just make it grayer and not really help at all.

How can I achieve this effect in lighter conditions?

Answer 1

What you are looking for is a ND (Neutral Density) filter.

To illustrate, here is an example of a photo taken in daylight in a street with a ND1000 filter. The filter allowed a shutter speed of 6 seconds. With no filter, with the same aperture and ISO, the shutter speed would have been approximately 6/1000 = 0.006 seconds (no "ghosts" effect).

Contrary to what you may think, a "good" ND filter is by definition "neutral". You will have exactly the same color with it, your sensor will simply need more time gathering light to produce an image with the same luminosity.

To put some science behind, basically, a ND filter is gray in appearance because gray absorb all radiations (all colors) more or less equally to the human eye.

You will find more information about how it works and its effects in the following posts :

• What are neutral density filters and how do I use them to create long exposures in daylight?
• What are the uses of Neutral Density (ND) filters?
• How do I practice with a ND filter?
• How does a variable ND filter work?

Answer 2

Apart from using an ND filter, you might be able to achieve the desired effect by taking multiple photos and then blending them in post processing. Either an automatic blend with "ghost removal" might work, or layering the images and manually masking/unmasking selectively (in effect "painting out" the people).

All of this pretty much requires a tripod for nicely aligned pictures, but that's the same for long exposures, of course.

Answer 3

Just to add a bit of links to the other (good) answers, if you do not want to use a ND filter, you can use multiple exposure and

1. use an averaging method to simulate a long exposure --- basically, 20 exposures at 1/10 of seconds will be more or less equivalent to a 2 seconds exposures, or

2. use a median filter, which can even be better --- in the right conditions, the people can just disappear.

PS --- I am not, nor related to, Pat David. I just like his tutorials.

Notice however that to have the same effect than a ND1000 filter with averaging, without changing the other exposure parameters (only the shutter time), you'll need 1000 exposures... which is around 1% of your average camera shutter life. If there aren't too much people around, you can go with much less shots in the median case; the trick here is to guess how much you need (partial success is not so nice as the "averaged ghosts").

Answer 4

As mentioned in Oliver's answer, you can use a neutral density filter, this let's through only a small fraction of the light, but it doesn't affect the color. You can then shoot at large aperture and yet have long exposure times.

Another solution is to take many pictures and then use image stacking methods. This method can be used under favorable conditions to completely remove all the moving objects. What you then do is you align the pictures and either use the median or remove the pixelwise maximum and minimum and take the average, or use some hybrid between these two extremes. If for each background point there is always an image for which that point in not obscured, it is in principle possible to reconstruct the scene with all the people removed. If the overlap between the obscured parts is typically not large, then it's easy to do automatically by taking the median or the average with the maximum and minimum image removed.

Answer 5

Another option is, if you're happy with the resolution your DSLR can create videos, is to record the scene for a few seconds (or even a few minutes), and later blur the frames together. Pros of this approach compared to the multiple image one is that you'll have usually much more frames to blur, and the result will be much smoother, the cons is that the image quality (especially resolution) won't be as good as with the single image approach.